title: Multigrid meets Neural Nets

abstract:
  We present evidence that multigrid (MG) works for wave equations in
disordered systems, e.g. in the presence of gauge fields, no matter how strong
the disorder. We introduce a "neural computations" point of view into large
scale simulations: First, the system must learn how to do the simulations
efficiently, then do the simulation (fast). The method can also be used to
provide smooth interpolation kernels which are needed in multigrid Monte Carlo
updates.


title: Decoupling of Photon Propagator in Compact QED

abstract:
  In compact QED$_{2+1}$ quantum monopole fluctuations induce confinement by
expelling electric flux in a dual Meissner effect. Guided by Landau-Ginzburg
theory, one might guess that the inverse London penetration depth
$\lambda^{-1}$---the only physical mass scale---equals the photon propagator
mass pole $M_\gamma$. I show this is not true. Indeed, in the Villain
approximation the monopole part of the partition function factorizes from the
photon part, whose dynamical variables are Dirac strings. Since Dirac strings
are gauge-variant structures, I conclude that $M_\gamma$ is physically
irrelevant: it is not a blood relative of $\lambda$ or any other quantity in
the gauge-invariant sector. This result is confirmed by numerical simulations
in the full theory, where $M_\gamma$ is not sensitive to monopole prohibition
but essentially vanishes if Dirac strings are prohibited.


title: Non-Relativistic QCD for Heavy Quark Systems

abstract:
  We employ a nonrelativistic version of QCD (NRQCD) to study heavy
quark-antiquark bound states in the lowest approximation without fine
structure. We use gluon configurations on a 16^3 by 48 lattice at beta=6.2 from
the UKQCD collaboration. For quark masses in the vicinity of the b we obtain
bound state masses for S, P and both types of D wave. We also detect signals
for two types of hybrids (quark,antiquark,gluon states). The results are
sufficiently accurate to confirm that the values of the D wave mass from both
lattice D waves coincide indicating that the cubical invariance of the lattice
is restored to full rotational invariance at large distance.
  Our results also show that the S-P splitting is indeed insensitive to
variations in the bare quark mass from Ma=1.0 to Ma=1.9.


title: Blockspin Scheme and Cluster Algorithm for Quantum Spin Systems

abstract:
  We present a numerical study using a cluster algorithm for the 1-d $S=1/2$
quantum Heisenberg models. The dynamical critical exponent for
anti-ferromagnetic chains is $z=0.0(1)$ such that critical slowing down is
eliminated.


title: Quark Confinement in Multi-Flavor Quantum Chromodynamics

abstract:
  It is investigated how quark confinement depends on the number of flavors,
$N_f$, in QCD on the lattice with Wilson quarks. We strengthen and extend the
conclusion reported at {\it Lattice 91}: (1) For $N_f \le 6$ the finite
temperature deconfining transition/crossover line crosses the chiral limit at
finite $\beta$. We identify the crossing point for $N_f=2$ and 6 on a $T=4$
lattice, where $T$ is the lattice size in the temporal direction. We find the
phase transition at the crossing point is continuous for $N_f=2$, while it is
of first order for $N_f=6$. (2) For $N_f \ge 7$, the $T$-independent
deconfining transition observed at $\beta=0$ extends up to $\beta=4.5$ with the
critical quark mass $m_{\rm quark} = O(a^{-1})$, where $a$ is the lattice
spacing.


title:
Baryon Density Correlations in the Quark Plasma, MIMD Lattice
  Calculation

abstract:
  As part of an ongoing effort to characterize the high temperature phase of
QCD, we measure the quark baryon density in the vicinity of a fixed test quark
and compare it with similar measurements at low temperature and at the
crossover temperature. Such an observable has also been studied by the Vienna
group. We find an extremely weak correlation at high temperature, suggesting
that small color singlet clusters are unimportant in the thermal ensemble. We
also find that at T = 0.75 T_c the induced quark number shows a surprisingly
large component attributable to baryonic screening. A simulation of a simple
flux tube model produces results that suggest a plausible scenario: As the
crossover temperature is approached from below, baryonic states proliferate.
Above the crossover temperature the mean size of color singlet clusters grows
explosively, resulting in an effective electrostatic deconfinement.


title: Improved Langevin Methods for Spin Systems

abstract:
  We investigate methods for variance reduction and the elimination of
systematic error in a Fourier accelerated Langevin scheme for general spin
models. We present results for the $SU(3)\times SU(3)/SU(3)$ model in
two-dimensions that are consistent wit h those from multi-grid methods. We
argue that the timing for the Langevin method makes it comparable to multi-grid
for a given level of error.


title: The Spin Content of the Nucleon

abstract:
  The fraction of the nucleon spin that is carried by the quarks, $\Delta
\Sigma$, is computed in lattice QCD with dynamical staggered fermions. We
obtain the value $\Delta \Sigma = 0.18 \pm 0.02$.


title:
Gauge Invariant Smearing and Matrix Correlators using Wilson Fermions at
  beta=6.2

abstract:
  We present an investigation of gauge invariant smearing for Wilson fermions
on a $24^3 \times 48$ lattice at $\beta = 6.2$. We demonstrate a smearing
algorithm that allows a substantial improvement in the determination of the
baryon spectrum obtained using propagators smeared at both source and sink, at
only a small computational cost. We investigate the matrix of correlators
constructed from local and smeared operators, and are able to expose excited
states of both the mesons and baryons.


title:
Quenched Light Hadron Spectroscopy: Comparing the Wilson and
  O(a)-Improved Fermion Actions

abstract:
  We have studied the light hadron spectrum and decay constants for quenched
QCD at beta=6.2 on a 24^3x48 lattice. We compare the results obtained using a
nearest-neighbour O(a)-improved ("clover") fermion action with those obtained
using the standard Wilson fermion action on the same gauge configurations. For
pseudoscalar meson masses in the range 330-800 MeV, we find no significant
difference between the results for the two actions. The scales obtained from
the string tension and mesonic sector are consistent, but higher than those
derived from baryon masses. The ratio of the pseudoscalar decay constant to the
vector meson mass is roughly independent of quark mass as observed
experimentally.


title: Results in the static approximation

abstract:
  We present a comprehensive study of finite volume and lattice spacing effects
on observables calculated in the static approximation. Ground state projection
using smearing techniques is studied at $\beta=6.26$. We give high statics
results for the pseudoscalar decay constant after extrapolating to the chiral
and continuum limit. In addition results for mass splittings and the distance
of string breaking in the $Q\bar{Q}$ potential are presented.


title:
The Static Approximation of Heavy-Light Quark-Systems - A Systematic
  Lattice Study

abstract:
  We present a study of {\it finite} $a$ and {\it volume} effects of the
leptonic decay constant $f$ of heavy pseudoscalar mesons in the static
approximation. This study is performed on a number of lattices at $\beta=$
5.74,~6.0 and 6.26 covering sizes from about 0.7~$fm$ to 2~$fm$. We confirm
that beyond 1.5~$fm$ the volume dependence is negligible. By carefully
analysing results obtained using different trial wave functions for the heavy
meson we find no dependence on the smearing. We also give results for the mass
difference of the scalar - pseudoscalar and the $\Lambda _{b}$ - pseudoscalar.
Using the mass of the pseudoscalar meson we estimate the distance of string
breaking in the static quark potential.


title: The Charm Quark on the Lattice

abstract:
  We formulate lattice fermions in a way that encompasses Wilson fermions as
well as the static and non-relativistic approximations. In particular, we treat
$m_qa$ systematically ($m_q$ is the fermion mass) showing how to understand the
Wilson action as an effective action for systems with $\vek{p}\ll m_q$. The
results show how to extract matrix elements and the spectrum from simulations
with $m_qa\approx1$, which is relevant for the charm quark.


title:
Gravitational Action Versus Entropy on Simplicial Lattices in Four
  Dimensions

abstract:
  We investigate quantum gravity on simplicial lattices using Regge calculus
with special emphasize on the problem of the unbounded action. The role of the
entropy for the path integral is discussed in detail. Our numerical results
show further evidence for the existence of an entropy dominated region with
well defined expectation values even for unbounded action. Analyses are
performed both for the standard regular triangulation of the 4-torus and for
irregularly triangulated lattices obtained by insertion of vertices using
barycentric subdivision.


title:
Charmed Meson Spectroscopy and Matrix Elements with an O(a)-Improved
  Clover Fermion Action

abstract:
  We present preliminary results for the spectrum and decay matrix elements for
heavy-light and heavy-heavy mesons, obtained on the 64-node Meiko Computing
Surface at the University of Edinburgh. Quark propagators are computed with an
O(a)-improved fermion action on 24^3x48 lattices at beta = 6.2, using three
values of the quark mass up to around the strange quark mass, and four values
of the quark mass in the region of the charm quark mass. We compare results for
the hyperfine splitting in charmonium with those obtained using the
conventional Wilson fermion action and find that the splitting is 1.83(15)
times larger with the improved action. Our measurements of f_B indicate
non-scaling corrections of the order of 20% to the Heavy Quark Effective Theory
expectation. A comparison is made with results obtained on 16^3x48 lattices at
beta = 6.0.


title: The Charmonium Spectrum on the Lattice: A Status Report

abstract:
  We present our most recent results on the charmonium spectrum using
relativistic Wilson fermions. We study the dependence of the spectrum on the
charm quark mass and the Wohlert-Sheikholeslami improvement term.


title: The Loop-Cluster Algorithm for the Case of the 6 Vertex Model

abstract:
  We present the loop algorithm, a new type of cluster algorithm that we
recently introduced for the F model. Using the framework of Kandel and Domany,
we show how to GENERALIZE the algorithm to the arrow flip symmetric 6 vertex
model. We propose the principle of least possible freezing as the guide to
choosing the values of free parameters in the algorithm. Finally, we briefly
discuss the application of our algorithm to simulations of quantum spin
systems. In particular, all necessary information is provided for the
simulation of spin $\half$ Heisenberg and $xxz$ models.


title: Quenched Lattice Results for $f_B$ and $f_D$

abstract:
  We have computed the decay constants for the $B$ and $D$ mesons, using
quenched lattices at $\beta=6.3$, by interpolating between the static
approximation of Eichten and the conventional (``heavy'' Wilson fermion)
method. A more careful treatment of the static result using better sources with
longer time-displacements and a modification to the Wilson quark normalization
to correct approximately for lattice effects in the large-$am$ regime have led
to the elimination of the large discrepancy between the two methods which had
been previously observed. We report final results, with estimates of various
systematic errors.


title: Multicanonical Cluster Algorithm

abstract:
  In this talk I present a multicanonical hybrid-like two-step algorithm, which
consists of a microcanonical spin system update with demons, and a
multicanonical demon refresh. The demons act as a buffer between the
multicanonical heat bath and the spin system, allowing for a large variety of
update schemes. In this work the cluster algorithm is demonstrated with the
2-dimensional 7-state Potts model, using volumes up to $128^2$.


title: The Mechanism of Complex Langevin Simulations

abstract:
  We discuss conditions under which expectation values computed from a complex
Langevin process $Z$ will converge to integral averages over a given complex
valued weight function. The difficulties in proving a general result are
pointed out. For complex valued polynomial actions, it is shown that for a
process converging to a strongly stationary process one gets the correct answer
for averages of polynomials if $c_{\tau}(k) \equiv E(e^{ikZ(\tau)}) $ satisfies
certain conditions. If these conditions are not satisfied, then the stochastic
process is not necessarily described by a complex Fokker Planck equation. The
result is illustrated with the exactly solvable complex frequency harmonic
oscillator.


title: Infinite Volume, Continuum Limit of Valence Approximation Hadron Masses

abstract:
  We obtain estimates of several hadron mass ratios, for Wilson quarks in the
valence (quenched) approximation, extrapolated to physical quark mass, infinite
volume and zero lattice spacing.


title:
Yet another way to obtain low temperature expansions for discrete spin
  systems

abstract:
  I present a modification of the shadow-lattice technique, which allows one to
derive low temperature series for discrete spin models to high orders. Results
are given for the 3-d Ising model up to 64 excited bonds, for the 4-d Ising
model up to 96 excited bonds and the 3-d Potts model up to 56 excited bonds.


title:
High precision single-cluster Monte Carlo measurement of the critical
  exponents of the classical 3D Heisenberg model

abstract:
  We report measurements of the critical exponents of the classical
three-dimensional Heisenberg model on simple cubic lattices of size $L^3$ with
$L$ = 12, 16, 20, 24, 32, 40, and 48. The data was obtained from a few long
single-cluster Monte Carlo simulations near the phase transition. We compute
high precision estimates of the critical coupling $K_c$, Binder's parameter
$U^* and the critical exponents $\nu,\beta / \nu, \eta$, and $\alpha / \nu$,
using extensively histogram reweighting and optimization techniques that allow
us to keep control over the statistical errors. Measurements of the
autocorrelation time show the expected reduction of critical slowing down at
the phase transition as compared to local update algorithms. This allows
simulations on significantly larger lattices than in previous studies and
consequently a better control over systematic errors in finite-size scaling
analyses.


title: Glueball wavefunctions

abstract:
  We measure Coulomb-gauge wavefunctions of the scalar and tensor glueballs in
SU(2) Yang-Mills. The problem of contamination by flux states is discussed, and
a new analysis method described. The large size of the tensor glueball is
confirmed. Preliminary results in the deconfined phase show no significant
changes.


title: Finite-Size Scaling on the Ising Coexistence Line

abstract:
  We study the finite-size scaling of moments of the magnetization in the
low-temperature phase of the two-dimensional Ising model.


title:
Study of the complex fermion determinant in a $\rm U(1)_L \otimes
  U(1)_R$ symmetric Yukawa model

abstract:
  Lattice theories that contain chiral multiplets of fermions can have complex
fermion determinants. This is for example the case for the $\rm U(1)_L \otimes
U(1)_R$ symmetric Yukawa model with mirror fermions, if the number of
generations of fermions and mirror fermions is odd. Whether a numerical
simulation of such a model is possible depends on the magnitude of fluctuations
of the complex phase factor of the fermion determinant. We investigate the
fermion determinant of the U(1) Yukawa model with mirror fermions for a
physically relevant choice of parameters. The argument of the complex phase
turns out to fluctuate only very little and is at most of the order of $2 \cdot
10^{-3}$.


title: Meson Form-factors and Wave-functions with Wilson fermions

abstract:
  Results for semi-leptonic form-factors for processes like $D \to K l \nu$ and
the Bethe-Salpeter amplitudes (BSA) for pion and rho mesons are presented. The
form-factor data is consistent with previous calculations. We find that the
long distance fall-off of BSA for both $\pi$ and $\rho$ is very well fit by an
exponential, but surprisingly the effective mass governing this fall-off is
lighter than the pion's. Lastly, by studying the dependence of $\rho$
polarization on separation direction we show that there is a measureable $l=2$
state in addition to $l=0$ in the BSA for the rho. (Talk presented by R. Gupta
at LATTICE92. Latex needs macro package espcrc2.sty)


title: Chiral Corrections to Lattice Calculations of Charge Radii

abstract:
  Logarithmic divergences in pion and proton charge radii associated with
chiral loops are investigated to assess systematic uncertainties in current
lattice determinations of charge radii. The chiral corrections offer a possible
solution to the long standing problem of why present lattice calculations yield
proton and pion radii which are similar in size.


title: Finite temperature QED3 with light fermions

abstract:
  Non-compact QED3 is simulated both in the quenched and unquenched cases. In
particular, we investigate the restoration of chiral symmetry at finite
temperature. We also compute the zero temperature spectrum of the theory,
including (in the quenched case) the dynamical fermion mass. From these two set
of data, one can obtain estimates for the ratio of the mass gap to the critical
temperature, of particular interest for applications to high-Tc
superconductivity.


title: Staggered fermion approach to chiral gauge theories on the lattice

abstract:
  The staggered fermion approach to build models with chiral fermions is
briefly reviewed. The method is tested in a U(1) model with axial vector
coupling in two and four dimensions.


title:
No strong coupling regime in the fermion-Higgs sector of the standard
  model

abstract:
  We present results for the renormalized quartic self-coupling $\lm_R$ and the
renormalized Yukawa coupling $y_R$ in a fermion-Higgs model with two SU(2)
doublets, indicating that these couplings are not very strong.


title:
Algebraic Techniques for Enumerating Self-Avoiding Walks on the Square
  Lattice

abstract:
  We describe a new algebraic technique for enumerating self-avoiding walks on
the rectangular lattice. The computational complexity of enumerating walks of
$N$ steps is of order $3^{N/4}$ times a polynomial in $N$, and so the approach
is greatly superior to direct counting techniques. We have enumerated walks of
up to 39 steps. As a consequence, we are able to accurately estimate the
critical point, critical exponent, and critical amplitude.


title:
Enumeration of self avoiding trails on a square lattice using a transfer
  matrix technique

abstract:
  We describe a new algebraic technique, utilising transfer matrices, for
enumerating self-avoiding lattice trails on the square lattice. We have
enumerated trails to 31 steps, and find increased evidence that trails are in
the self-avoiding walk universality class. Assuming that trails behave like $A
\lambda ^n n^{11 \over 32}$, we find $\lambda = 2.72062 \pm 0.000006$ and $A =
1.272 \pm 0.002$.


title: Fermion mass in the Wilson-Yukawa approach for chiral Yukawa theory

abstract:
  We consider a modification of the Wilson-Yukawa model to overcome its
difficulty that the fermion mass is not proportional to the Higgs vacuum
expectation value. In the modification scalar and fermionic regulator fields
are introduced so that all the physical fermion fields possess shift symmetry
when the Yukawa coupling vanishes. With the fermionic hopping parameter
expansion it is shown that the fermion mass is proportional to the Higgs vacuum
expectation value and that the coupling of fermion to the external gauge field
can be chiral if the hopping parameter of the scalar regulator field is taken
to the critical value.


title:
The S, U and \Delta\rho parameters in the Zaragoza proposal for lattice
  chiral gauge fermions

abstract:
  Using the Zaragoza proposal for lattice chiral gauge fermions, the S, U and
\Delta\rho parameters have been calculated at one loop. It is shown that the
continuum values for these quantities can be reproduced without requiring
explicit fine tuning of counterterms. Furthermore, fermion fields doubling is
not necessary. To the best of our knowledge, the Zaragoza proposal is the only
scheme which has these properties. A necessary (although not sufficient)
symmetry is found to support the calculations. Previous results for some of
these parameters in other lattice chiral regularizations are revisited in the
light of this symmetry.


title:
Running Coupling in pure gauge theories using the Schr\"odinger
  functional

abstract:
  Schr\"odinger functional, the propagation kernel for going from some field
configuration at time $x^0=0$ to some other configuration at $x^0=T$, is used
to define a running coupling, $\bar g^2(L)$, at a length scale, $L$, in pure
gauge theories. Using a lattice formulation and finite size scaling techniques,
this running coupling is calculated non-perturbatively in the continuum for a
wide range of $L$, that extends from the perturbative scales to
non-perturbative scales, for the specific case of pure SU(2) gauge theory.


title: Chiral fermions in two dimensions?

abstract:
  Quenched studies of a global U(1) symmetric Wilson-Yukawa model in two
dimensions show no evidence of a charged fermion in the vortex phase at strong
Wilson-Yukawa coupling while there is strong indication of a massive neutral
fermion. However, with the U(1)$_L$ gauge field turned on, we use dimensional
arguments to suggest that the neutral fermion appears to couple chirally to a
massive vector boson state.


title: Dirac Strings and the Nonperturbative Photon Propagator in Compact QED

abstract:
  A ``3D'' Mathematica@-generated picture of monopoles and Dirac strings in
$D=2+1$ compact QED is given. In the Villain approximation, the monopole part
of the partition function factorizes from the Dirac string part, which
generates the photon propagator. Numerical experiments in exact compact QED
confirm this result: photon mass pole $M_\gamma$, originally nonzero, is
insensitive to monopole prohibition but almost vanishes if Dirac strings are
prohibited.


title:
Kazakov-Migdal Induced Gauge Theory and the Coupling of 2D Quantum
  Gravity to D=1 Matter

abstract:
  Recently Kazakov and Migdal proposed a new approach to the large $N$ limit of
SU(N) gauge theories which could hopefully describe the asymptotically free
fixed point of QCD in 4 dimensions. In this contribution we review the exact
solution of their model in the case of a $d=1$ compactified lattice and its
connection with the partition function of the vortex-free sector of the d=1
compactified bosonic string. Some new results on the addition of a linear term
in the K-M model and on the SU(N) restriction of the general U(N) model are
also presented.


title: Magnetic Monopoles in non-compact QED - is there a Phase Transition?

abstract:
  The existence of the monopole condensation transition reported by
  Kocic et al. in non-compact, quenched QED is tested. No phase transition is
found. This shows that divergence of the `monopole susceptibility' introduced
by Hands and Wensley is not a reliable indicator of second order phase
transitions. In view of these results
  I discuss claims that the chiral phase transition seen in QED with fermions
is a lattice artefact driven by monopole condensation.


title: Gauge fixing and Gribov copies in pure Yang-Mills on a circle

abstract:
  %In order to understand how gauge fixing can be affected on the %lattice, we
first study a simple model of pure Yang-mills theory on a %cylindrical
spacetime [$SU(N)$ on $S^1 \times$ {\bf R}] where the %gauge fixed subspace is
explicitly displayed. On the way, we find that %different gauge fixing
procedures lead to different Hamiltonians and %spectra, which however coincide
under a shift of states. The lattice %version of the model is compared and
lattice gauge fixing issues are %discussed. (---TALK GIVEN AT LATTICE
92---AMSTERDAM, 15 SEPT. 92)


title: The Crumpling Transition Revisited

abstract:
  The ``crumpling" transition, between rigid and crumpled surfaces, has been
object of much discussion over the past years. The common lore is that such
transition should be of second order. However, some lattice versions of the
rigidity term on fixed connectivity surfaces seem to suggest that the
transition is of higher order instead. While some models exhibit what appear to
be lattice artifacts, others are really indistiguishable from models where
second order transitions have been reported and yet appear to have third order
transitions.


title:
INvestigation Of The Biconjugate Gradient Algorithm For The Inversion Of
  Fermion Matrices

abstract:
  An algorithm for the numerical inversion of large matrices, the biconjugate
gradient algorithm (BGA), is investigated in view of its use for Monte Carlo
simulations of fermionic field theories. It is compared with the usual
conjugate gradient algorithm (CGA) and the minimal residue algorithm (MRA)
within a Higgs-Yukawa model including mirror fermions. For this model it can be
shown that BGA represents an improvement under certain circumstances where the
other two slow down.


title:
Renormalization Group Flow in non-compact QED with two charged staggered
  Fermions

abstract:
  We have investigated a system with two sets of staggered fermions with
charges 1 and -1/2 coupling to a non-compact U(1) gauge field in 4 dimensions.
The model exhibits breaking of chiral symmetries of both fermions at different
values of beta. Chiral condensates, renormalized fermion masses and
renormalized charges have been measured. The renormalized charges show
agreement with one-loop perturbation theory. We examine surfaces of constant
renormalized charges in the space of bare parameters.


title:
Autocorrelation in Updating Pure SU(3) Lattice Gauge Theory by the use
  of Overrelaxed Algorithms

abstract:
  We measure the sweep-to-sweep autocorrelations of blocked loops below and
above the deconfinement transition for SU(3) on a $16^4$ lattice using
20000-140000 Monte-Carlo updating sweeps. A divergence of the autocorrelation
time toward the critical $\beta$ is seen at high blocking levels. The peak is
near $\beta$ = 6.33 where we observe 440 $\pm$ 210 for the autocorrelation time
of $1\times 1$ Wilson loop on $2^4$ blocked lattice. The mixing of 7 Brown-Woch
overrelaxation steps followed by one pseudo-heat-bath step appears optimal to
reduce the autocorrelation time below the critical $\beta$. Above the critical
$\beta$, however, no clear difference between these two algorithms can be seen
and the system decorrelates rather fast.


title: The Euclidean Spectrum of Kaplan's Lattice Chiral Fermions

abstract:
  We consider the (2n+1)-dimensional euclidean Dirac operator with a mass term
that looks like a domain wall, recently proposed by Kaplan to describe chiral
fermions in $2n$ dimensions. In the continuum case we show that the euclidean
spectrum contains {\it no} bound states with non-zero momentum. On the lattice,
a bound state spectrum without energy gap exists only if $m$ is fine tuned to
some special values, and the dispersion relation does not describe a
relativistic fermion. In spite of these peculiarities, the fermionic propagator
{\it has} the expected (1/p-slash) pole on the domain wall. But there may be a
problem with the phase of the fermionic determinant at the non-perturbative
level.


title: Phase Structure of Compact $QED_3$ with Massless Fermions

abstract:
  In the framework of (2+1)-dimensional compact lattice QED with light
fermions, we investigate the phase diagram in the $(\beta, N)$ plane. The
approximations involved are related to an expansion of the effective fermionic
action as a power series of the flavor number $N$. We also develop a new
mechanism for understanding the $N-$critical phenomenon in the full theory. Our
results for the specific heat indicate that only one phase does exist. We give
strong evidences that this qualitative result should not be changed with the
inclusion of higher order terms in the $N$ expansion.


title:
Possible Failure of Asymptotic Freedom in Two-Dimensional $RP^2$ and
  $RP^3$ $\sigma$-Models

abstract:
  We have simulated the two-dimensional $RP^2$ and $RP^3$ $\sigma$-models, at
correlation lengths up to about 220 (resp.\ 30), using a Wolff-type embedding
algorithm. We see no evidence of asymptotic scaling. Indeed, the data rule out
the conventional asymptotic scaling scenario at all correlation lengths less
than about $10^{9}$ (resp.\ $10^{5}$). Moreover, they are consistent with a
critical point at $\beta \approx 5.70$ (resp.\ 6.96), only 2\% (resp.\ 5\%)
beyond the largest $\beta$ at which we ran. Preliminary studies of a mixed
$S^{N-1}/RP^{N-1}$ model (i.e. isovector + isotensor action) show a similar
behavior when $\beta_T \to \infty$ with $\beta_V$ fixed $\ltapprox 0.6$, while
they are consistent with conventional asymptotic freedom along the lines
$\beta_T/\beta_V$ fixed $\ltapprox 2$. Taken as a whole, the data cast doubt on
(though they do not completely exclude) the idea that $RP^{N-1}$ and $S^{N-1}$
$\sigma$-models lie in the same universality class.


title: Finite-Size Effects in the Interface of 3D Ising Model

abstract:
  The interface between domains of opposite magnetization in the 3D Ising model
near the critical temperature displays universal finite-size effects which can
be described in terms of a gaussian model of capillary waves. It turns out that
these finite-size corrections depend rather strongly on the shape of the
lattice. This prediction, which has no adjustable parameters, is tested and
accurately verified for various lattice shapes by means of numerical
simulations with a cluster algorithm. This supports also a long-standing
conjecture on the finite-size effects in Wilson loops of Lattice Gauge
Theories.


title: A New Action for Heavy Lattice Fermions

abstract:
  I describe a unified formalism for lattice fermions, in which the
relativistic action of Wilson and the nonrelativistic and static actions appear
as special cases. It is valid at all values of $m_q a$, including $m_q a
\approx 1$. In the limit $m_q a \ll 1 $, the formulation reduces to the light
quark action of Wilson. In the limit $m_q a \gg 1 $, the formulation reduces to
the nonrelativistic action of Thacker and Lepage, and to the static action of
Eichten.


title:
Mass Spectrum and Bounds on the Couplings in Yukawa Models With
  Mirror-Fermions

abstract:
  The $\rm SU(2)_L\otimes SU(2)_R$ symmetric Yukawa model with mirror-fermions
in the limit where the mirror-fermion is decoupled is studied both analytically
and numerically. The bare scalar self-coupling $\lambda$ is fixed at zero and
infinity. The phase structure is explored and the relevant phase transition is
found to be consistent with a second order one. The fermionic mass spectrum
close to that transition is discussed and a first non-perturbative estimate of
the influence of fermions on the upper and lower bounds on the renormalized
scalar self-coupling is given. Numerical results are confronted with
perturbative predictions.


title: Baryon Octet to Decuplet Electromagnetic Transitions

abstract:
  The electromagnetic transition moments of the $SU(3)$-flavor baryon octet to
decuplet are examined within a lattice simulation of quenched QCD. The magnetic
transition moment for the $N \; \gamma \to \Delta$ channel is found to be in
agreement with recent experimental analyses. The lattice results indicate
$\mu_{p \Delta} / \mu_p = 0.88(15)$. In terms of the Particle Data Group
convention, $f_{M1} = 0.231(41)$ GeV${}^{-1/2}$ for $p \; \gamma \to \Delta^+$
transitions. Lattice predictions for the hyperon $M1$ transition moments agree
with those of a simple quark model. However the manner in which the quarks
contribute to the transition moments in the lattice simulation is different
from that anticipated by quark model calculations. The scalar quadrupole form
factor exhibits a behavior consistent with previous multipole analyses. The
$E2/M1$ multipole transition moment ratios are also determined. The lattice
results suggest $R_{EM} \equiv -{\cal G}_{E2}/{\cal G}_{M1} = +3\pm 8$ \% for
$p \; \gamma \to \Delta^+$ transitions. Of particular interest are significant
nonvanishing signals for the $E2/M1$ ratio in $\Xi^-$ and $\Sigma^-$
electromagnetic transitions.


title: Progress in Lattice Field Theory Algorithms

abstract:
  I present a summary of recent algorithmic developments for lattice field
theories. In particular I give a pedagogical introduction to the new
Multicanonical algorithm, and discuss the relation between the Hybrid
Overrelaxation and Hybrid Monte Carlo algorithms. I also attempt to clarify the
role of the dynamical critical exponent z and its connection with
`computational cost.' [Includes four PostScript figures]


title: Heavy Quark Physics

abstract:
  Lattice calculations of heavy quark systems provide very good measures of the
lattice spacing, a key element in recent determinations of the strong coupling
constant using lattice methods. They also provide excellent testing grounds for
lattice methods in general. I review recent phenomenological and technical
developments in this field.


title: Infinitely many regulator fields for chiral fermions

abstract:
  We show that two recent independent proposals for regularizing a chiral gauge
theory stem from one common trick. If the anomaly free complex representation
carried by the right handed fermi--fields is $r$ one constructs a vector like
theory with flavored right handed fermionic matter in $r+\bar r$ but with a
mass matrix of the order of the cutoff and having an index equal to unity in an
infinite dimensional flavor space. We present a Pauli--Villars realization of
the trick that is likely to work to all orders in perturbation theory and a
lattice version which is argued to produce the correct continuum leading order
fermionic contribution to the vacuum polarization tensor and readied for
further perturbative checks.


title: Large N Lattice QED

abstract:
  We study the $\beta, N$ critical behaviour of non compact QED with $N$
species of light fermions, using a method we have proposed for unquenched
simulations. We find that there exist two phase transition lines: one, second
order, and the other, first order, that approaches asymptotically the $\beta=0$
axis. These two lines have different physical origin, the second one being
entirely due to fermions effects. We discuss the effect of the approximation
used, in terms of an expansion of the effective action in powers of $N$, and
conclude that the general features should not be affected by this
approximation.


title: Multigrid Methods for the Computation of Propagators in Gauge Fields

abstract:
  NOTE: this is a shortened version of the abstract of the paper.
  Multigrid methods for propagators in gauge fields are investigated.
  Gauge fields are incorporated in algorithms in a covariant way. This avoids
the necessity for gauge fixing in computations of propagators.
  The kernel $C$ of the restriction operator which averages from one grid to
the next coarser grid is defined by projection on the ground- state of a local
Hamiltonian. The idea behind this definition is that the appropriate notion of
smoothness depends on the dynamics. The ground-state projection choice of $C$
is usable in arbitrary space-time dimension $d$ and for arbitrary gauge group.
We discuss proper averaging operations for bosons and for staggered fermions.
The averaging kernels $C$ can be used not only in deterministic multigrid
computations, but also in multigrid Monte Carlo simulations, and for the
definition of block spins and blocked gauge fields in Monte Carlo
renormalization group studies of gauge theories. Actual numerical computations
of kernels and propagators are performed in compact four-dimensional $SU(2)$
gauge fields. A central result of the present work is that {\em the multigrid
method works in arbitrarily disordered gauge fields, in principle\/}.


title: Finite size and temperature effects in the AF Heisenberg model

abstract:
  The low temperature and large volume effects in the d=2+1 antiferromagnetic
quantum Heisenberg model are dominated by magnon excitations. The leading and
next-to-leading corrections are fully controlled by three physical constants,
the spin stiffness, the spin wave velocity and the staggered magnetization.
Among others, the free energy, the ground state energy, the low lying
excitations, staggered magnetization, staggered and uniform susceptibilities
are studied here. The special limits of very low temperature and infinite
volume are considered also.


title: Cosmic Strings on the Lattice

abstract:
  We develop a formalism for the quantization of topologically stable
excitations in the 4-dimensional abelian lattice gauge theory. The excitations
are global and local (Abrikosov-Nielsen-Olesen) strings and monopoles. The
operators of creation and annihilation of string states are constructed; the
string Green functions are represented as a path integral over random surfaces.
Topological excitations play an important role in the early universe. In the
broken symmetry phase of the $U(1)$ spin model, closed global cosmic strings
arise, while in the Higgs phase of the noncompact gauge-Higgs model, local
cosmic strings are present. The compact gauge-Higgs model also involves
monopoles. Then the strings can break if their ends are capped by monopoles.
The topology of the Euclidean string world sheets are studied by numerical
simulations.


title: How much are 2d Yukawa models similar to the Gross-Neveu models?

abstract:
  We present numerical evidence that the 2d Yukawa models with strong quartic
selfcoupling of the scalar field have the same phase structure and are
asymptotically free in the Yukawa coupling like the Gross-Neveu models.


title:
The Shifted Coupled Cluster Method: A New Approach to Hamiltonian
  Lattice Gauge Theories

abstract:
  It is shown how to adapt the non-perturbative coupled cluster method of
many-body theory so that it may be successfully applied to Hamiltonian lattice
$SU(N)$ gauge theories. The procedure involves first writing the wavefunctions
for the vacuum and excited states in terms of linked clusters of gauge
invariant excitations of the strong coupling vacuum. The fundamental
approximation scheme then consists of i) a truncation of the infinite set of
clusters in the wavefunctions according to their geometric {\em size}, with all
larger clusters appearing in the Schr\"odinger equations simply discarded, ii)
an expansion of the truncated wavefunctions in terms of the remaining clusters
rearranged, or ``shifted'', to describe gauge invariant {\em fluctuations}
about their vacuum expectation values. The resulting non-linear truncated
Schr\"odinger equations are then solved self-consistently and exactly. Results
are presented for the case of $SU(2)$ in $d=3$ space-time dimensions.


title:
Can the Couplings in the Fermion-Higgs Sector of the Standard Model be
  Strong?

abstract:
  We present results for the renormalized quartic self-coupling $\lambda_R$ and
the Yukawa coupling $y_R$ in a lattice fermion-Higgs model with two SU(2)$_L$
doublets, mostly for large values of the bare couplings. One-component
(`reduced') staggered fermions are used in a numerical simulation with the
Hybrid Monte Carlo algorithm. The fermion and Higgs masses and the renormalized
scalar field expectation value are computed on $L^3 24$ lattices, where $L$
ranges from $6$ to $16$. In the scaling region these quantities are found to
have a $1/L^2$ dependence, which is used to determine their values in the
infinite volume limit. We then calculate the $y_R$ and $\lambda_R$ from their
tree level definitions in terms of the masses and renormalized scalar field
expectation value, extrapolated to infinite volume. The scalar field
propagators can be described for momenta up to the cut-off by one fermion loop
renormalized perturbation theory and the results for $\lambda_R$ and $y_R$ come
out to be close to the tree level unitarity bounds. There are no signs that are
in contradiction with the triviality of the Yukawa and quartic self-coupling.


title: The Spectrum of the Kazakov Migdal-Model

abstract:
  Gross has found an exact expression for the density of eigenvalues in the
simplest version of the Kazakov-Migdal model of induced QCD. In this paper we
compute the spectrum of small fluctuations around Gross's semi-circular
solution. By solving Migdal's wave equation we find a string-like spectrum
which, in four dimensions, corresponds to the infinite tower of mesons in
strong coupling lattice QCD with adjoint matter. In one dimension our formula
reproduces correctly the well known spectrum of the hermitean matrix model with
a harmonic oscillator potential. We comment on the relevance of our results to
the possibility of the model describing extended objects in more than one
dimension.


title: Large scale numerical simulation of the three-state Potts model

abstract:
  The three-state Potts model is numerically investigated on three-dimensional
simple cubic lattices of up to \(128^3\) volume, concentrating on the
neighborhood of the first-order phase transition separating the ordered and
disordered phases. The ordered phase is found to allow admixture of disordered
domains induced by a long-range attraction acting between the two different
non-favored spins. This phenomenon gives an explanation of why the first-order
phase transitions associated with the global \(Z_3\) symmetry are so weak.
(Talk given at the International Symposium "Lattice 92" in Amsterdam, September
15-19, 1992)


title: Strongly Interacting Higgs Sector in the Minimal Standard Model?

abstract:
  The triviality Higgs mass bound is studied {\it without} lattice regulator in
the spontaneously broken phase of the four dimensional O(4) symmetric scalar
field theory with quartic self-interaction. A higher derivative term is
introduced in the kinetic energy of the Lagrangian to keep quantum fluctuations
finite while preserving all the symmetries of the model. When viewed as a {\it
finite} field theory in continuum space-time, some excitations of the Higgs
field have to be quantized with indefinite metric in the Hilbert space. It is
shown that the associated ghost particles, which exhibit unusual resonance
properties, correspond to a complex conjugate pair of Pauli-Villars regulator
masses in the euclidean path integral formulation. The phase diagram of the
O(4) model is determined in a Monte Carlo simulation which interpolates between
the hypercubic lattice regulator and the higher derivative regulator in
continuum space-time. The same method can be used to calculate the triviality
Higgs mass bound without lattice artifacts. In a large-N analysis, when
compared with a hypercubic lattice, we find a relative increase in the
triviality bound of the higher derivative regulator suggesting a strongly
interacting Higgs


title: Chiral Fermions, Anomalies and Chern-Simons Currents on the Lattice

abstract:
  I discuss the zeromode spectrum of lattice chiral fermions in the domain wall
model suggested recently. In particular I give the critical momenta where the
fermions cease to be chiral and show that the spectrum is directly related to
the behaviour of the Chern-Simons current on the lattice. First results for the
domain wall model on the finite lattice indicate that the relevant features of
the model in the infinite system survive for the finite lattice.


title: Hadron Mass Predictions of the Valence Approximation to Lattice QCD

abstract:
  We evaluate the infinite volume, continuum limits of eight hadron mass ratios
predicted by lattice QCD with Wilson quarks in the valence (quenched)
approximation. Each predicted ratio differs from the corresponding observed
value by less than 6\%.


title: Series studies of the Potts model. I: The simple cubic Ising model

abstract:
  The finite lattice method of series expansion is generalised to the $q$-state
Potts model on the simple cubic lattice.
  It is found that the computational effort grows exponentially with the square
of the number of series terms obtained, unlike two-dimensional lattices where
the computational requirements grow exponentially with the number of terms. For
the Ising ($q=2$) case we have extended low-temperature series for the
partition functions, magnetisation and zero-field susceptibility to $u^{26}$
from $u^{20}$. The high-temperature series for the zero-field partition
function is extended from $v^{18}$ to $v^{22}$. Subsequent analysis gives
critical exponents in agreement with those from field theory.


title: A realistic heat bath: theory and application to kink-antikink dynamics

abstract:
  We propose a new method of studying a real-time canonical evolution of
field-theoretic systems with boundary coupling to a realistic heat bath. In the
free-field case the method is equivalent to an infinite extension of the system
beyond the boundary, while in the interacting case the extension of the system
is done in linear approximation. We use this technique to study kink-antikink
dynamics in $\varphi^4$ field theory in 1+1 dimensions.


title: On the Dynamics of Light Quarks in QCD

abstract:
  We describe recent results concerning the behavior of lattice QCD with light
dynamical Wilson and Staggered quarks. We show that it is possible to reach
regions of parameter space with light pions $m_\pi\approx 0.2/a$ using Wilson
fermions. If the Hybrid Molecular Dynamics (HMD) algorithm is used with the
same parameters it gives incorrect results. We also present preliminary results
using a higher-order integration scheme.


title: Calculating the Isgur-Wise Function on the Lattice

abstract:
  We calculate the Isgur-Wise function by measuring the heavy-heavy meson
transition matrix element on the lattice. The standard Wilson action is used
for both the heavy and light quarks. Our first numerical results are presented.


title: Heavy-Light Wavefunctions in Lattice QCD

abstract:
  Using a multistate smearing method, Coulomb gauge wave functions of
heavy-light mesons are studied in lattice QCD. Wave functions for the ground
state, the first radially excited S-wave state, and the lowest P-wave states of
a heavy-light meson are calculated in quenched approximation. The results are
found to be in remarkably good agreement with the predictions of a simple
relativistic quark model.


title:
Critical Exponents of the Classical 3D Heisenberg Model: A
  Single-Cluster Monte Carlo Study

abstract:
  We have simulated the three-dimensional Heisenberg model on simple cubic
lattices, using the single-cluster Monte Carlo update algorithm. The expected
pronounced reduction of critical slowing down at the phase transition is
verified. This allows simulations on significantly larger lattices than in
previous studies and consequently a better control over systematic errors. In
one set of simulations we employ the usual finite-size scaling methods to
compute the critical exponents $\nu,\alpha,\beta,\gamma, \eta$ from a few
measurements in the vicinity of the critical point, making extensive use of
histogram reweighting and optimization techniques. In another set of
simulations we report measurements of improved estimators for the spatial
correlation length and the susceptibility in the high-temperature phase,
obtained on lattices with up to $100^3$ spins. This enables us to compute
independent estimates of $\nu$ and $\gamma$ from power-law fits of their
critical divergencies.


title: Measuring the Decorrelation Times of Fourier Modes in Simulations

abstract:
  We describe a method to study the rate at which modes decorrelate in
numerical simulations. We study the XY model updated with the Metropolis and
Wolff dynamics respectively and compute the rate at which each eigenvector of
the dynamics decorrelates. Our method allows us to identify the decorrelation
time for each mode separately. We find that the autocorrelation function of the
various modes is markedly different for the `local' Metropolis compared to the
`non-local' Wolff dynamics. Equipped with this new insight, it may be possible
to devise highly efficient algorithms.


title: Structure of Abrikosov Vortices in SU(2) Lattice Gauge Theory

abstract:
  We calculate the electric flux and magnetic monopole current distribution in
the presence of a static quark-antiquark pair for SU(2) lattice gauge theory in
the maximal Abelian gauge. The current distribution confines the flux in a dual
Abrikosov vortex whose core size is comparable to the flux penetration depth.
The observed structure is described by a dual Ginzburg-Landau model.


title: The Heavy Quark Self-Energy in Nonrelativistic Lattice QCD

abstract:
  The heavy quark self-energy in nonrelativistic lattice QCD is calculated to
$O(\alpha_s)$ in perturbation theory. An action which includes all
spin-independent relativistic corrections to order $v^2$, where $v$ is the
typical heavy quark velocity, and all spin-dependent corrections to order $v^4$
is used. The standard Wilson action and an improved multi-plaquette action are
used for the gluons. Results for the mass renormalization, wavefunction
renormalization, and energy shift are given; tadpole contributions are found to
be large. A tadpole improvement scheme in which all link variables are rescaled
by a mean-field factor is also studied. The effectiveness of this scheme in
offsetting the large tadpole contributions to the heavy quark renormalization
parameters is demonstrated.


title: Multi-Quark Energies in SU(2) Lattice Gauge Theory

abstract:
  Energies of four-quark systems have been extracted in a quenched SU(2)
lattice Monte Carlo calculation for two different geometries, rectangular and
colinear, with $\beta=2.4$ and lattice size $16^3\times 32$. Also, by going to
a lattice $24^3\times 32$ and to $\beta=2.5$, the effect of the finite lattice
size and scaling are checked. An attempt is made to understand these results in
terms of a model based on interquark two-body potentials but modified very
significantly by a phenomenological gluon-field overlap factor.


title: On the Crumpling Transition in Crystalline Random Surfaces

abstract:
  We investigate the crumpling transition on crystalline random surfaces with
extrinsic curvature on lattices up to $64^2$. Our data are consistent with a
second order phase transition and we find correlation length critical exponent
$\nu=0.89\pm 0.07$. The specific heat exponent, $\alpha=0.2\pm 0.15$, is in
much better agreement with hyperscaling than hitherto. The long distance
behaviour of tangent-tangent correlation functions confirms that the so-called
Hausdorff dimension is $d_H=\infty$ throughout the crumpled phase.


title: Finite size scaling in CP(N-1) models

abstract:
  Finite size effects in Euclidean ${\rm CP}^{N-1}$ models with periodic
boundary conditions are investigated by means of the $1/N$ expansion and by
Monte Carlo simulations. Analytic and numerical results for magnetic
susceptibility and correlation length are compared and found to agree for small
volumes. For large volumes a discrepancy is found and explained as an effect of
the physical bound state extension. The leading order finite size effects on
the Abelian string tension are computed and compared with simulations finding
agreement. Finite size dependence of topological quantities is also discussed.


title:
Does the 2d Higgs-Yukawa Model Have a Symmetric Phase at Small Yukawa
  Coupling Region?

abstract:
  We show that at arbitrary value of the scalar self coupling and small Yukawa
coupling $y$ the 2d Higgs-Yukawa model with Z(2) symmetry remains in the broken
phase and the model is asymptotically free: $y \to 0$ as the cut-off $\Lambda
\to \infty$. This is in agreement with a recent conjecture based on numerical
simulation results.


title: Noncompact Lattice Simulations of SU(2) Gauge Theory

abstract:
  Wilson loops have been measured at strong coupling, $\beta=0.5$, on a $12^4$
lattice in noncompact simulations of pure SU(2) without gauge fixing. There is
no sign of quark confinement.


title: Multigrid Monte Carlo with higher cycles in the Sine Gordon model

abstract:
  We study the dynamical critical behavior of multigrid Monte Carlo for the two
dimensional Sine Gordon model on lattices up to 128 x 128. Using piecewise
constant interpolation, we perform a W-cycle (gamma=2). We examine whether one
can reduce critical slowing down caused by decreasing acceptance rates on large
blocks by doing more work on coarser lattices. To this end, we choose a higher
cycle with gamma = 4. The results clearly demonstrate that critical slowing
down is not reduced in either case.


title:
Lattice Perturbation Theory by Computer Algebra: A Three-Loop Result for
  the Topological Susceptibility

abstract:
  We present a scheme for the analytic computation of renormalization functions
on the lattice, using a symbolic manipulation computer language. Our first
nontrivial application is a new three-loop result for the topological
susceptibility.


title:
Landau, Abrikosov, Hofstadter: Magnetic Flux Penetration in a Lattice
  Superconductor

abstract:
  Magnetic flux penetration in superconductors involves a rich variety of
subtle phenomena, much of which is still poorly understood. Here these
complexities are studied by formulating the Ginzburg-Landau equations as a
lattice gauge theory. Their solutions are compared and contrasted with the
(heuristic) Landau model of type I superconductivity, and the (perturbative)
Abrikosov model for type II superconductors. Novelties arise as the continuum
limit is approached, related to an effect discovered by Hofstadter. Various
cautionary remarks pertinent to large-scale simulations are made.


title:
SU(N) Quantum Antiferromagnets and the Phase Structure of QED in the
  Strong Coupling Limit

abstract:
  We examine the strong coupling limit of both compact and non compact QED on a
lattice with staggered fermions. We show that every SU(N) antiferromagnet with
spins in a particular fundamental representation of the SU(N) Lie Algebra and
with nearest neighbor couplings on a bipartite lattice is exactly equivalent to
the infinite coupling limit of lattice QED with the numbers of flavors of
electrons related to N and the dimension of spacetime D+1. We find that,for
both compact and noncompact QED,when N is odd the ground state of the strong
coupling limit breaks chiral symmetry in any dimensions and for any N and the
condensate is an isoscalar mass operator. When N is even,chiral symmetry is
broken if D is bigger or equal to 2 and N is small enough and the order
parameter is an isovector mass operator. We also find the exact ground state of
the lattice Coulomb gas as well as a variety of related lattice statistical
systems with long ranged interactions.


title: Chiral Gauge Theories and Fermion-Higgs Systems

abstract:
  Summary talk presented at the Conference on Lattice Field Theory, Amsterdam,
September, 1992. Abstract: The status of several proposals for defining a
theory of chiral fermions on the lattice is reviewed and some new estimates for
the upper bound on the Higgs mass are presented.


title: $I=2$ pion scattering amplitude with Wilson fermions

abstract:
  We present an exploratory calculation of the $I=2$ $\pi-\pi$ scattering
amplitude at threshold using Wilson fermions in the quenched approximation,
including all the required contractions. We find good agreement with the
predictions of chiral perturbation theory even for pions of mass 560-700 MeV.
Within the 10\% errors, we do not see the onset of the bad chiral behavior
expected for Wilson fermions. We also derive rigorous inequalities that apply
to 2-particle correlators and as a consequence show that the interaction in the
antisymmetric state of two pions has to be attractive.


title: Low Temperature Expansions for Potts Models

abstract:
  On simple cubic lattices, we compute low temperature series expansions for
the energy, magnetization and susceptibility of the three-state Potts model in
D=2 and D=3 to 45 and 39 excited bonds respectively, and the eight-state Potts
model in D=2 to 25 excited bonds. We use a recursive procedure which enumerates
states explicitly. We analyze the series using Dlog Pade analysis and
inhomogeneous differential approximants.


title:
Simple Hadronic Matrix Elements with Wilson Valence Quarks and Dynamical
  Staggered Fermions at ${\bf 6/g^2=5.6}$

abstract:
  We have measured some simple matrix elements for pseudoscalar and vector
mesons made of Wilson valence quarks and staggered sea quarks at $\beta=5.6$ at
sea quark masses $am_q=0.01$ and 0.025. Our measurements include the decay
constants of pseudoscalars (including $f_D$), the wave function at the origin
(or decay constant) of vector mesons, and the calculation of quark masses from
current algebra. The effects of sea quarks on the simulations are small. We
make comparisons to quenched simulations at similar values of the lattice
spacing ($1/a \simeq 2$ GeV).


title: Lattice study of sphaleron transitions in a 2D O(3) sigma model

abstract:
  A lattice approach is developed to measure the sphaleron free energy. Its
feasibility is demonstrated through a Monte Carlo study of the two-dimensional
O(3) sigma model.


title:
The Confined-Deconfined Interface Tension in Quenched QCD using the
  Histogram Method

abstract:
  We present results for the confinement-deconfinement interface tension
$\sigma_{cd}$ of quenched QCD. They were obtained by applying Binder's
histogram method to lattices of size $L^2\times L_z\times L_t$ for $L_t=2$ and
$L=8,10,12\mbox{ and }14$ and various $L_z\in [L,\, 4\, L]$. The use of a
multicanonical algorithm and rectangular geometries have turned out to be
crucial for the numerical studies. We also give an estimate for $\sigma_{cd}$
at $L_t=4$ using published data.


title:
Charge Screening, Large-N, and the Abelian Projection Model of
  Confinement

abstract:
  We point out that the abelian projection theory of quark confinement is in
conflict with certain large-N predictions. According to both large-N and
lattice strong-coupling arguments, the perimeter law behavior of adjoint Wilson
loops at large scales is due to charge-screening, and is suppressed relative to
the area term by a factor of $1/N^2$. In the abelian projection theory,
however, the perimeter law is due to the fact that $N-1$ out of $N^2-1$ adjoint
quark degrees of freedom are (abelian) neutral and unconfined; the suppression
factor relative to the area law is thus only $1/N$. We study numerically the
behavior of Wilson loops and Polyakov lines with insertions of (abelian) charge
projection operators, in maximal abelian gauge. It appears from our data that
the forces between abelian charged, and abelian neutral adjoint quarks are not
significantly different. We also show via the lattice strong-coupling expansion
that, at least at strong couplings, QCD flux tubes attract one another, whereas
vortices in type II superconductors repel.


title: Recent Developments in Lattice QCD

abstract:
  I review the current status of lattice QCD results. I concentrate on new
analytical developments and on numerical results relevant to phenomenology.


title: Steiner Variations on Random Surfaces

abstract:
  Ambartzumian et.al. suggested that the modified Steiner action functional had
desirable properties for a random surface action. However, Durhuus and Jonsson
pointed out that such an action led to an ill-defined grand-canonical partition
function and suggested that the addition of an area term might improve matters.
In this paper we investigate this and other related actions numerically for
dynamically triangulated random surfaces and compare the results with the
gaussian plus extrinsic curvature actions that have been used previously.


title: Recycle of random sequences

abstract:
  The correlation between a random sequence and its transformed sequences is
studied. In the case of a permutation operation or, in other word, the
shuffling operation, it is shown that the correlation can be so small that the
sequences can be regarded as independent random sequences. The applications to
the Monte Carlo simulations are also given. This method is especially useful in
the Ising Monte Carlo simulation.


title: New Lower Bounds on the Self-Avoiding-Walk Connective Constant

abstract:
  We give an elementary new method for obtaining rigorous lower bounds on the
connective constant for self-avoiding walks on the hypercubic lattice $Z^d$.
The method is based on loop erasure and restoration, and does not require exact
enumeration data. Our bounds are best for high $d$, and in fact agree with the
first four terms of the $1/d$ expansion for the connective constant. The bounds
are the best to date for dimensions $d \geq 3$, but do not produce good results
in two dimensions. For $d=3,4,5,6$, respectively, our lower bound is within
2.4\%, 0.43\%, 0.12\%, 0.044\% of the value estimated by series extrapolation.


title:
Renormalization and topological susceptibility on the lattice: SU(2)
  Yang-Mills theory

abstract:
  The renormalization functions involved in the determination of the
topological susceptibility in the SU(2) lattice gauge theory are extracted by
direct measurements, without relying on perturbation theory. The determination
exploits the phenomenon of critical slowing down to allow the separation of
perturbative and non-perturbative effects. The results are in good agreement
with perturbative computations.


title: Vacuum Tunneling and Periodic Structure in Lattice Higgs Models

abstract:
  Using a geometric definition for the lattice Chern-Simons term in even
dimensions, we have studied the distribution of Chern-Simons numbers for the
2d-U(1) and the 4d-SU(2) lattice Higgs models. The periodic structure of the
distributions is preserved in our lattice formulation and has been examined in
detail. In both cases the finite size effects visible in the distribution of
Chern-Simons numbers are well accounted for by the Haar measure. Moreover, we
find that $\langle N_{CS}^2 \rangle$ grows with the spatial volume. We also
find numerical evidence that tunneling in 4d is increased at high temperature.
(PS-File including Figures available via E-mail: plache@physv.uni-bielefeld.de)


title:
Numerical Study of Periodic Instanton Configurations in Two-dimensional
  Abelian Higgs Theory

abstract:
  Numerical minimization of the Euclidean action of the two-dimensional Abelian
Higgs model is used to construct periodic instantons, the euclidean field
configurations with two turning points describing transitions between the
vicinities of topologically distinct vacua. Periodic instantons are found at
any energy ( up to the sphaleron energy $E_{sph}$ ) and for wide range of
parameters of the theory. We obtain the dependence of the action and the energy
of periodic instanton on its period; these quantities directly determine the
probability of certain multiparticle scattering events.


title:
Instanton-like Contributions to the Dynamics of Yang-Mills Fields on the
  Twisted Torus

abstract:
  We study $SU(2)$ lattice gauge theory in small volumes and with twist
$\vec{m}=(1,1,1)$. We investigate the presence of the periodic instantons of
$Q=\frac{1}{2}$ and determine their free energy and their contribution to the
splitting of energy flux sectors \mbox{$E(\vec{e}=(1,1,1))-E(\vec{e}=(0,0,0))
$.}


title:
Prediction of Peptide Conformation by Multicanonical Algorithm: A New
  Approach to the Multiple-Minima Problem

abstract:
  We apply a recently developed method, multicanonical algorithm, to the
problem of tertiary structure prediction of peptides and proteins. As a simple
example to test the effectiveness of the algorithm, Met-enkephalin is studied
and the ergodicity problem, or multiple-minima problem, is shown to be overcome
by this algorithm. The lowest-energy conformation obtained agrees with that
determined by other efficient methods such as Monte Carlo simulated annealing.
The superiority of the present method to simulated annealing lies in the fact
that the relationship to the canonical ensemble remains exactly controlled.
Once the multicanonical parameters are determined, only one simulation run is
necessary to obtain the lowest-energy conformation and furthermore the results
of this one run can be used to calculate various thermodynamic quantities at
any temperature. The latter point is demonstrated by the calculation of the
average potential energy and specific heat as functions of temperature.


title: SU(2) Flux Distributions on Finite Lattices

abstract:
  We studied SU(2) flux distributions on four dimensional euclidean lattices
with one dimension very large. By choosing the time direction appropriately we
can study physics in two cases: one is finite volume in the zero temperature
limit, another is finite temperature in the the intermediate to large volume
limit. We found that for cases of beta > beta crit there is no intrinsic string
formation. Our lattices with beta > beta crit belong to intermediate volume
region, and the string tension in this region is due to finite volume effects.
In large volumes we found evidence for intrinsic string formation.


title:
Measurement of the penetration depth and coherence length in U(1) and
  SU(2) dual Abrikosov vortices

abstract:
  We calculate the electric field and the curl of the magnetic monopole current
for U(1) and for SU(2) in the maximal abelian gauge in the mid- plane between a
quark antiquark pair. The results can be understood as a dual Abrikosov vortex
in the Ginzburg-Landau theory.


title: Vacuum structure of pure gauge theories on the lattice

abstract:
  We present results from simulations on two aspects of quark confinement in
the pure gauge sector. First is the calculation of the profile of the flux tube
connecting a static $q \bar{q}$ pair in $SU(2)$. By using the Michael sum rules
as a constraint we give evidence that the energy density at the center of the
flux tube goes to a constant as a function of quark separation. Slow variation
of the width and energy density is not ruled out. Secondly in the confined
phase of lattice $U(1)$ we calculate the curl of the magnetic monopole current
and show that the dual London equation is satisfied and that the electric
fluxoid is quantized.


title:
Meson Decay Constant Predictions of the Valence Approximation to Lattice
  QCD

abstract:
  We evaluate $f_{\pi}/ m_{\rho}$, $f_K/ m_{\rho}$, $1/f_{\rho}$, and $
m_{\phi}/(f_{\phi} m_{\rho})$, extrapolated to physical quark mass, zero
lattice spacing and infinite volume, for lattice QCD with Wilson quarks in the
valence (quenched) approximation. The predicted $m_{\phi}/(f_{\phi} m_{\rho})$
differs from experiment by less than its statistical uncertainty of
approximately 15\%. The other three constants are 10\% to 20\% below
experiment, equivalent to between one and two times the corresponding
statistical uncertainties.


title:
Is the Chiral Phase Transition in Non-Compact Lattice QED Driven by
  Monopole Condensation?

abstract:
  We investigate the recent conjecture that the chiral phase transition in
non-compact lattice QED is driven by monopole condensation. The comparison of
analytic and numerical results shows that we have a quantitative understanding
of monopoles in both the quenched and dynamical cases. We can rule out monopole
condensation.


title: Series expansions without diagrams

abstract:
  We discuss the use of recursive enumeration schemes to obtain low and high
temperature series expansions for discrete statistical systems. Using linear
combinations of generalized helical lattices, the method is competitive with
diagramatic approaches and is easily generalizable. We illustrate the approach
using the Ising model and generate low temperature series in up to five
dimensions and high temperature series in three dimensions. The method is
general and can be applied to any discrete model. We describe how it would work
for Potts models.


title: Unstable Modes in Three-Dimensional SU(2) Gauge Theory

abstract:
  We investigate SU(2) gauge theory in a constant chromomagnetic field in three
dimensions both in the continuum and on the lattice. Using a variational method
to stabilize the unstable modes, we evaluate the vacuum energy density in the
one-loop approximation. We compare our theoretical results with the outcomes of
the numerical simulations.


title: Removing fermion doublers in chiral gauge theories on the lattice

abstract:
  A method for removing fermion doublers in anomaly free chiral gauge theories
on the lattice is proposed. It is proven that the resulting continuum theory is
gauge invariant and does not require noninvariant counterterms of fine tuning
of parameters.


title: Chiral Fermions from Lattice Boundaries

abstract:
  We construct a model in which four dimensional chiral fermions arise on the
boundaries of a five dimensional lattice with free boundary conditions in the
fifth direction. The physical content is similar to Kaplan's model of domain
wall fermions, yet the present construction has several technical advantages.
We discuss some aspects of perturbation theory, as well as possible
applications of the model both for lattice QCD and for the on-going attempts to
construct a lattice chiral gauge theory.


title: Smooth Random Surfaces from Tight Immersions?

abstract:
  We investigate actions for dynamically triangulated random surfaces that
consist of a gaussian or area term plus the {\it modulus} of the gaussian
curvature and compare their behavior with both gaussian plus extrinsic
curvature and ``Steiner'' actions.


title:
String Representation of the Abelian Higgs Theory and Aharonov-Bohm
  Effect on the Lattice

abstract:
  The partition function of the $4D$ lattice Abelian Higgs theory is
represented as the sum over world sheets of Nielsen--Olesen strings. The
creation and annihilation operators of the strings are constructed. The
topological long--range interaction of the strings and charged particles is
shown to exist; it is proportional to the linking number of the string world
sheet and particle world trajectory.


title: Flux-tubes in three-dimensional lattice gauge theories

abstract:
  Flux-tubes in different representations of SU(2) and U(1) lattice gauge
theories in three dimensions are measured. Wilson loops generate heavy
``quark-antiquark'' pairs in fundamental ($j=1/2$), adjoint ($j=1$), and
quartet ($j=3/2$) representations of SU(2). The first direct lattice
measurements of the flux-tube cross-section ${\cal A}_j$ as a function of
representation are made. It is found that ${\cal A}_j \approx {\rm constant}$,
to about 10\%. Results are consistent with a connection between the string
tension $\sigma_j$ and ${\cal A}_j$ suggested by a simplified flux-tube model,
$\sigma_j = g^2 j(j+1) / (2 {\cal A}_j)$ [$g$ is the gauge coupling], given
that $\sigma_j$ scales like the Casimir $j(j+1)$, as observed in previous
lattice studies in both three and four dimensions. The results can discriminate
among phenomenological models of the physics underlying confinement. Flux-tubes
for singly- and doubly-charged Wilson loops in compact QED$_3$ are also
measured. It is found that the string tension scales as the squared-charge and
the flux-tube cross-section is independent of charge to good approximation.
These SU(2) and U(1) simulations lend some support, albeit indirectly, to a
conjecture that the dual superconductor mechanism underlies confinement in
compact gauge theories in both three and four dimensions.


title:
Gauge-Invariant Smearing and Matrix Correlators using Wilson Fermions at
  beta=6.2

abstract:
  We present an investigation of gauge-invariant smearing for Wilson fermions
in quenched lattice QCD on a $24^3 \times 48$ lattice at $\beta = 6.2$. We
demonstrate a smearing algorithm that allows a substantial improvement in the
determination of the baryon spectrum obtained using propagators smeared at both
source and sink, at only a small computational cost. We investigate the matrix
of correlators constructed from local and smeared operators, and are able to
expose excited states of both the mesons and baryons.


title: Study of the Asymptotic Freedom of 2D Yukawa Models on the Lattice

abstract:
  We investigate on the lattice the Yukawa models in 2 dimensions with Z(2) and
U(1) symmetries. These models reduce to the usual and chiral Gross-Neveu
models, respectively, when the kinetic and the selfcoupling terms of the scalar
field are turned off. The numerical data and mean field arguments suggest that,
at least for some range of the scalar field hopping parameter, fermion mass is
dynamically generated for arbitrarily weak Yukawa coupling. The models are
asymptotically free in this coupling, like the Gross-Neveu models, even when
the scalar quartic selfcoupling is strong.


title:
Time-dependent correlation functions in a one-dimensional asymmetric
  exclusion process

abstract:
  We study a one-dimensional anisotropic exclusion process describing particles
injected at the origin, moving to the right on a chain of $L$ sites and being
removed at the (right) boundary. We construct the steady state and compute the
density profile, exact expressions for all equal-time n-point density
correlation functions and the time-dependent two-point function in the steady
state as functions of the injection and absorption rates. We determine the
phase diagram of the model and compare our results with predictions from
dynamical scaling and discuss some conjectures for other exclusion models.


title:
Bounds on the renormalized couplings in an SU(2)_L \otimes SU(2)_R
  symmetric Yukawa model

abstract:
  The vacuum stability lower bound on the mass of the Higgs boson is
numerically investigated in an $SU(2)_L \otimes SU(2)_R$ symmetric Yukawa
model, which describes two heavy degenerate fermion doublets in the limit of
vanishing gauge couplings. Good agreement with perturbation theory is found,
although the couplings are strong. The upper bound on the fermion mass and
renormalized Yukawa coupling is also determined in the part of bare parameter
space where reflection positivity has been proven.


title:
Nonequilibrium Dynamics and Aging in the Three--Dimensional Ising Spin
  Glass Model

abstract:
  The low temperature dynamics of the three dimensional Ising spin glass in
zero field with a discrete bond distribution is investigated via MC
simulations. The thermoremanent magnetization is found to decay algebraically
and the temperature dependent exponents agree very well with the experimentally
determined values. The nonequilibrium autocorrelation function $C(t,t_w)$ shows
a crossover at the waiting (or {\em aging}) time $t_w$ from algebraic {\em
quasi-equilibrium} decay for times $t$$\ll$$t_w$ to another, faster algebraic
decay for $t$$\gg$$t_w$ with an exponent similar to one for the remanent
magnetization.


title: The Coleman-Weinberg Mechanism and First Order Phase Transitions

abstract:
  Revised and reduced the size of the text. Removed figure 2 in previous text.
So, old figure 3 and 4 become new figure 2 and 3, respectively. Added figure 4
for first order phase transition observation in the strong coupling region.
Postscript files for the figures are available upon request. To be published in
Phys. Lett. B.


title: The dimer partition function

abstract:
  We apply the Ginzburg criterion to the dimer problem and we solve the
apparent contradiction of a system with mean field $\alpha={1\over2}$, the
typical value of tricritical systems, and upper critical dimension $D_{cr}=6$.
We find that the system has upper critical dimension $D_{cr}=6$ , while for
$D\le4$ it should undergo a first order phase transition. We comment on the
latter wrong result examining the approximation we used.


title:
Critical behaviour of the 1D q-state Potts model with long-range
  interactions

abstract:
  The critical behaviour of the one-dimensional q-state Potts model with
long-range interactions decaying with distance r as $r^{-(1+\sigma)}$ has been
studied in the wide range of parameters $0 < \sigma \le 1$ and $\frac{1}{16}
\le q \le 64$. A transfer matrix has been constructed for a truncated range of
interactions for integer and continuous q, and finite range scaling has been
applied. Results for the phase diagram and the correlation length critical
exponent are presented.


title: Heavy Dynamical Fermions in Lattice QCD

abstract:
  It is expected that the only effect of heavy dynamical fermions in QCD is to
renormalize the gauge coupling. We derive a simple expression for the shift in
the gauge coupling induced by $N_f$ flavors of heavy fermions. We compare this
formula to the shift in the gauge coupling at which the
confinement-deconfinement phase transition occurs (at fixed lattice size) from
numerical simulations as a function of quark mass and $N_f$. We find remarkable
agreement with our expression down to a fairly light quark mass. However,
simulations with eight heavy flavors and two light flavors show that the eight
flavors do more than just shift the gauge coupling. We observe
confinement-deconfinement transitions at $\beta=0$ induced by a large number of
heavy quarks. We comment on the relevance of our results to contemporary
simulations of QCD which include dynamical fermions.


title: Comment on ``Antiferromagnetic Potts Models''

abstract:
  We show that the Wang-Swendsen-Koteck\'y algorithm for antiferromagnetic
$q$-state Potts models is nonergodic at zero temperature for $q=3$ on periodic
$3m \times 3n$ lattices where $m,n$ are relatively prime. For $q \ge 4$ and/or
other lattice sizes or boundary conditions, the ergodicity at zero temperature
is an open question.


title: Charged fermion states in the quenched U(1) chiral Wilson-Yukawa model

abstract:
  Property of charged fermion states is investigated in the quenched U(1)
chiral Wilson-Yukawa model. Fitting the charged fermion propagator with a
single hyperbolic cosine does not yield reliable results. On the other hand the
behavior of the propagator including large lattice size dependence is well
described by the large Wilson-Yukawa coupling expansion, providing strong
evidence that no charged fermion state exists as an asymptotic particle in this
model.


title: Idealized Multigrid Algorithm for Staggered Fermions

abstract:
  An idealized multigrid algorithm for the computation of propagators of
staggered fermions is investigated.
  Exemplified in four-dimensional $SU(2)$ gauge fields, it is shown that the
idealized algorithm preserves criticality under coarsening.
  The same is not true when the coarse grid operator is defined by the Galerkin
prescription.
  Relaxation times in computations of propagators are small, and critical
slowing is strongly reduced (or eliminated) in the idealized algorithm.
  Unfortunately, this algorithm is not practical for production runs, but the
investigations presented here answer important questions of principle.


title: Analytic Variational Investigation of Euclidean SU(3) Gauge Theory

abstract:
  Analytic variational techniques for lattice gauge theories based on the
Rayleigh-Ritz(RR) method were previously developed for euclidean SU(2) gauge
theories in 3 and 4 dimensions. Their extensions to SU(3) gauge theory
including applications to correlation functions and mass gaps are presented
here.


title: Computing Masses from Effective Transfer Matrices

abstract:
  We study the use of effective transfer matrices for the numerical computation
of masses (or correlation lengths) in lattice spin models. The effective
transfer matrix has a strongly reduced number of components. Its definition is
motivated by a renormalization group transformation of the full model onto a
1-dimensional spin model. The matrix elements of the effective transfer matrix
can be determined by Monte Carlo simulation. We show that the mass gap can be
recovered exactly from the spectrum of the effective transfer matrix. As a
first step towards application we performed a Monte Carlo study for the
2-dimensional Ising model. For the simulations in the broken phase we employed
a multimagnetical demon algorithm. The results for the tunnelling correlation
length are particularly encouraging.


title: Critical Exponents of the Three Dimensional Random Field Ising Model

abstract:
  The phase transition of the three--dimensional random field Ising model with
a discrete ($\pm h$) field distribution is investigated by extensive Monte
Carlo simulations. Values of the critical exponents for the correlation length,
specific heat, susceptibility, disconnected susceptibility and magnetization
are determined simultaneously via finite size scaling. While the exponents for
the magnetization and disconnected susceptibility are consistent with a first
order transition, the specific heat appears to saturate indicating no latent
heat. Sample to sample fluctuations of the susceptibilty are consistent with
the droplet picture for the transition.


title: A Comparative Study of Some Pseudorandom Number Generators

abstract:
  We present results of an extensive test program of a group of pseudorandom
number generators which are commonly used in the applications of physics, in
particular in Monte Carlo simulations. The generators include public domain
programs, manufacturer installed routines and a random number sequence produced
from physical noise. We start by traditional statistical tests, followed by
detailed bit level and visual tests. The computational speed of various
algorithms is also scrutinized. Our results allow direct comparisons between
the properties of different generators, as well as an assessment of the
efficiency of the various test methods. This information provides the best
available criterion to choose the best possible generator for a given problem.
However, in light of recent problems reported with some of these generators, we
also discuss the importance of developing more refined physical tests to find
possible correlations not revealed by the present test methods.


title:
Bethe-Salpeter Amplitudes and Density Correlations for Mesons with
  Wilson Fermions

abstract:
  We present an investigation of various gauge invariant definitions of the
$q\bar q$ Bethe-Salpeter (BS) amplitude for mesons in lattice QCD, and compare
them to the Coulomb and Landau gauge BS amplitudes. We show that the gauge
invariant BS amplitude is considerably broadened by the use of ``fat'' gauge
links (constructed by smearing the links of the original lattice). A
qualitative demonstration of the Lorentz contraction of the pion wavefunction
at non-zero momentum is given. We also calculate density-density correlations
and discuss the limitations in extracting the charge radius of the pion from
these observables. Lastly, the polarization dependence of the BS amplitude for
the $\rho$ meson is exhibited, and we extract the relative admixture of $l=0$
and $l=2$ states showing that simple hydrogen-like wavefunctions provide a good
fit.


title: Numerical Simulations in Particle Physics

abstract:
  Numerical simulations have become an important tool to understand and predict
non-perturbative phenomena in particle physics. In this article we attempt to
present a general overview over the field. First, the basic concepts of lattice
gauge theories are described, including a discussion of currently used
algorithms and the reconstruction of continuum physics from lattice results. We
then proceed to present some results for QCD, both at low energies and at high
temperatures, as well as for the electro-weak sector of the standard model.


title: Lattice Calculation of Glueball Matrix Elements

abstract:
  Matrix elements of the form $ <0| Tr \; g^{2}GG |G> $ are calculated using
the lattice QCD Monte Carlo method. Here, $|G>$ is a glueball state with
quantum numbers $ 0^{++}$, $ 2^{++}$, $ 0^{-+}$ and $G$ is the gluon field
strength operator. The matrix elements are obtained from the hybrid correlation
functions of the fuzzy and plaquette operators performed on the $12^{4}$ and
$14^{4}$ lattices at $\beta = 5.9 $ and $5.96$ respectively. These matrix
elements are compared with those from the QCD sum rules and the tensor meson
dominance model. They are the non-perturbative matrix elements needed in the
calculation of the partial widths of $J/\Psi$ radiative decays into glueballs.


title: A comprehensive lattice study of SU(3) glueballs

abstract:
  We present a study of the $SU(3)$ glueball spectrum for all $J^{PC}$ values
at lattice spacings down to $a^{-1}=3.73 (6)$ GeV ($\beta=6.4$) using lattices
of size up to $32^4$. We extend previous studies and show that the continuum
limit has effectively been reached. The number of clearly identified $J^{PC}$
states has been substantially increased. There are no clear signals for
spin-exotic glueballs below 3 GeV. A comparison with current experimental
glueball candidates is made.


title: O(a) Perturbative improvement for Wilson fermions

abstract:
  The coefficient of the O(a)-improved
  Sheikholeslami-Wohlert action for Wilson fermions are perturbatively
determined at one-loop level and estimated at two-loop level.


title: Coloured Polymers

abstract:
  We show that non-oriented coloured polymers (self--avoiding walks with
different types of links) are in the same universality class of the ordinary
self--avoiding walks, while the oriented coloured are not.


title: The XY Model on a Dynamical Random Lattice

abstract:
  We study the XY model on a lattice with fluctuating connectivity. The
expectation is that at an appropriate critical point such a system corresponds
to a compactified boson coupled to 2d quantum gravity. Our simulations focus,
in particular, on the important topological features of the system. The results
lend strong support to the two phase structure predicted on the basis of
analytical calculations. A careful finite size scaling analysis yields
estimates for the critical exponents in the low temperature phase.


title: Three-Dimensional Quantum Gravity Coupled to Gauge Fields

abstract:
  We show how to simulate U(1) gauge fields coupled to three-dimensional
quantum gravity and then examine the phase diagram of this system. Quenched
mean field theory suggests that a transition separates confined and deconfined
phases (for the gauge matter) in both the negative curvature phase and the
positive curvature phase of the quantum gravity, but numerical simulations find
no evidence for such transitions.


title:
SU(2) QCD in the Path Representation: General Formalism and Mandelstam
  Indentities

abstract:
  We introduce a path-dependent hamiltonian representation (the path
representation) for SU(2) with fermions in 3 + 1 dimensions. The
gauge-invariant operators and hamiltonian are realized in a Hilbert space of
open path and loop functionals. We obtain two new types of Mandelstam
identities one that connects open path operators with loop operators and other
involving the end points of the paths.


title:
Critical exponents of the three-dimensional classical plane rotator
  model on the sc lattice from a high temperature series analysis

abstract:
  High temperature series expansions of the spin-spin correlation function for
the plane rotator (or XY) model on the sc lattice are extended by three terms
through order $\beta^{17}$. Tables of the expansion coefficients are reported
for the correlation function spherical moments of order $l=0,1,2$. Our analysis
of the series leads to fairly accurate estimates of the critical parameters.


title: The Higgs Model with a Complex Ghost Pair

abstract:
  A higher derivative term is introduced in the kinetic energy of the Higgs
Lagrangian in the minimal Standard Model. A logically consistent and {\it
finite} field theory is obtained when some excitations of the Higgs field are
quantized with indefinite metric in the Hilbert space. The Landau ghost
phenomenon of the conventional triviality problem is replaced by the state
vectors of a complex ghost pair at a finite mass scale with observable physical
consequences. It is shown that the ghost states exhibit unusual resonance
properties and correspond to a complex conjugate pair of Pauli-Villars
regulator masses in the euclidean path integral formulation of the theory. An
argument is given that microscopic acausality effects associated with the ghost
pair remain undetectable in scattering processes with realistic wave packects,
and the S-matrix should exhibit unitarity in the observable sector of the
Hilbert space. Part One of Extended UCSD-PTH 92-40


title: Strongly Interacting Higgs Sector in the Minimal Standard Model?

abstract:
  The triviality Higgs mass bound is studied with a higher derivative regulator
in the spontaneously broken phase of the four dimensional O(4) symmetric scalar
field theory with quartic self-interaction. The phase diagram of the O(4) model
is determined in a Monte Carlo simulation which interpolates between the
hypercubic lattice regulator and the higher derivative regulator in continuum
space-time. The same method can be used to calculate the Higgs mass bound in
continuum space-time. In a large-N analysis, when compared with a hypercubic
lattice, we find a relative increase in the triviality bound of the higher
derivative regulator suggesting a strongly interacting Higgs sector in the TeV
region with negligible dependence on regulator parameters. When the higher
derivative regulator mass is brought close to the Higgs mass the model requires
a more elaborate analysis of complex ghost states in scattering amplitudes.
Part Two of Extended UCSD-PTH 92-40


title:
High-Temperature Series Analyses of the Classical Heisenberg and XY
  Model

abstract:
  Although there is now a good measure of agreement between Monte Carlo and
high-temperature series expansion estimates for Ising ($n=1$) models, published
results for the critical temperature from series expansions up to 12{\em th}
order for the three-dimensional classical Heisenberg ($n=3$) and XY ($n=2$)
model do not agree very well with recent high-precision Monte Carlo estimates.
In order to clarify this discrepancy we have analyzed extended high-temperature
series expansions of the susceptibility, the second correlation moment, and the
second field derivative of the susceptibility, which have been derived a few
years ago by L\"uscher and Weisz for general $O(n)$ vector spin models on
$D$-dimensional hypercubic lattices up to 14{\em th} order in $K \equiv J/k_B
T$. By analyzing these series expansions in three dimensions with two different
methods that allow for confluent correction terms, we obtain good agreement
with the standard field theory exponent estimates and with the critical
temperature estimates from the new high-precision MC simulations. Furthermore,
for the Heisenberg model we reanalyze existing series for the susceptibility on
the BCC lattice up to 11{\em th} order and on the FCC lattice up to 12{\em th}
order.


title: Finite-Size Scaling at Phase Coexistence

abstract:
  {}From a finite-size scaling (FSS) theory of cumulants of the order parameter
at phase coexistence points, we reconstruct the scaling of the moments.
Assuming that the cumulants allow a reconstruction of the free energy density
no better than as an asymptotic expansion, we find that FSS for moments of low
order is still complete. We suggest ways of using this theory for the analysis
of numerical simulations. We test these methods numerically through the scaling
of cumulants and moments of the magnetization in the low-temperature phase of
the two-dimensional Ising model. (LaTeX file; ps figures included as shar file)


title: The $1/N$ expansion of two-dimensional spin models

abstract:
  A general two-dimensional spin model with U$(N)$ invariance, interpolating
between $\CPN$ and ${\rm O}(2N)$ models, is studied in detail in order to
illustrate both the general features of the $1/N$ expansion on the lattice and
the specific techniques devised to extract scaling (field-theoretical)
behavior. The continuum version of the model is carefully analyzed deriving
quantitative $O(1/N)$ physical predictions in order to establish a benchmark
for lattice computations. The $1/N$ expansion on the lattice, including
second-nearest-neighbor interactions, is set up by constructing explicitly
effective propagators and vertices. The technique of asymptotic expansion of
the lattice propagators, basic to the derivation of analytical results in the
scaling domain, is presented in full detail and applied to the model. Physical
quantities, like the free energy and different definitions of correlation
length, are evaluated. The lattice renormalization-group trajectories are
identified and universality among different lattice (and continuum) schemes in
the scaling region is explicitly proven. A review of other developments based
on the lattice $1/N$ expansion is presented.


title: On the Logarithmic Triviality of Scalar Quantum Electrodynamics

abstract:
  Using finite size scaling and histogram methods we obtain numerical results
from lattice simulations indicating the logarithmic triviality of scalar
quantum electrodynamics, even when the bare gauge coupling is chosen large.
Simulations of the non-compact formulation of the lattice abelian Higgs model
with fixed length scalar fields on $L^{4}$ lattices with $L$ ranging from $6$
through $20$ indicate a line of second order critical points.
Fluctuation-induced first order transitions are ruled out. Runs of over ten
million sweeps for each $L$ produce specific heat peaks which grow
logarithmically with $L$ and whose critical couplings shift with $L$ picking
out a correlation length exponent of $0.50(5)$ consistent with mean field
theory. This behavior is qualitatively similar to that found in pure
$\lambda\phi^{4}$.


title:
Static Scaling Behavior of High-Molecular-Weight Polymers in Dilute
  Solution: A Reexamination

abstract:
  Previous theories of dilute polymer solutions have failed to distinguish
clearly between two very different ways of taking the long-chain limit: (I) $N
\to\infty$ at fixed temperature $T$, and (II) $N \to\infty$, $T \to T_\theta$
with $x \equiv N^\phi (T-T_\theta)$ fixed. I argue that the modern
two-parameter theory (continuum Edwards model) applies to case II --- not case
I --- and in fact gives exactly the crossover scaling functions for $x \ge 0$
modulo two nonuniversal scale factors. A Wilson-type renormalization group
clarifies the connection between crossover scaling functions and continuum
field theories. [Also contains a general discussion of the connection between
the Wilson and field-theoretic renormalization groups. Comments solicited.]


title: Phases of the three-state Potts model in three spatial dimensions

abstract:
  The three-state Potts model is numerically investigated on three-dimensional
simple cubic lattices of up to \(128^3\) volume, concentrating on the
neighborhood of the first-order phase transition separating the ordered and
disordered phases. In both phases clusters of like spins are observed with
irregular boundaries. In the ordered phase the two different non-favored spins
are found to attract each other with a long but finite range. As a result, the
neighborhoods of the non-favored spins are interpreted as domains of the
disordered phase. This explains why the first-order phase transitions
associated with the global \(Z_3\) symmetry, including the SU(3) pure-gauge
one, are so weak.


title: The Coulomb-Higgs phase transition in Z(8) and q=8 U(1)-Higgs models

abstract:
  The Coulomb-Higgs phase transition of the four dimensional Z$_8$ gauge model
is studied. We find clear first order properties in contradiction with the
previously stressed second order behavior. That transition point may be
regarded as the end of a transition line of the U(1)-Higgs model with charge
$q=8$, that has been also assumed of second order. We show that this line is
first order in a wide region, with a decreasing latent heat that goes to zero
when closing to the 4--$d$ XY model limit.


title: Freezing a Fluid Random Surface

abstract:
  We investigate a dynamically triangulated random surface action that consists
of a gaussian term plus the modulus of the intrinsic scalar curvature. We find
that the flips are frozen out and the internal geometry is regularized as the
coefficient of the latter term is increased. Such a term thus provides a way of
interpolating between dynamically triangulated (ie fluid) and crystalline
random surfaces.


title: SU(2) finite temperature phase transition and Michael sum rules

abstract:
  We studied the finite temperature phase transition of SU(2) gauge theory on
four-dimensional Euclidean lattices by Monte Carlo simulations, and measured
the flux distributions of a $q\bar q$ pair in both confined and unconfined
phases. We reviewed and generalized Michael sum rules to include finite
temperature effects. To compare our flux data with predictions of Michael sum
rules, we studied the behavior of string tension with temperature. Our data
agree well with the generalized sum rules.


title: Undesirable effects of covariance matrix techniques for error analysis

abstract:
  Regression with $\chi^2$ constructed from the covariance matrix should not be
used for some combinations of covariance matrices and fitting functions. Using
the technique for unsuitable combinations can amplify systematic errors. This
amplification is uncontrolled, and can produce arbitrarily inaccurate results
that might not be ruled out by a $\chi^2$ test. In addition, this technique can
give incorrect (artificially small) errors for fit parameters. I give a test
for this instability and a more robust (but computationally more intensive)
method for fitting correlated data.


title: Phase Transitions in Two-Dimensional Traffic Flow Models

abstract:
  We introduce two simple two-dimensional lattice models to study traffic flow
in cities. We have found that a few basic elements give rise to the
characteristic phase diagram of a first-order phase transition from a freely
moving phase to a jammed state, with a critical point. The jammed phase
presents new transitions corresponding to structural transformations of the
jam. We discuss their relevance in the infinite size limit.


title: Multicanonical Multigrid Monte Carlo

abstract:
  To further improve the performance of Monte Carlo simulations of first-order
phase transitions we propose to combine the multicanonical approach with
multigrid techniques. We report tests of this proposition for the
$d$-dimensional $\Phi^4$ field theory in two different situations. First, we
study quantum tunneling for $d = 1$ in the continuum limit, and second, we
investigate first-order phase transitions for $d = 2$ in the infinite volume
limit. Compared with standard multicanonical simulations we obtain improvement
factors of several resp. of about one order of magnitude.


title:
Fermionic effective action and the pahse estructure of non compact
  quantum electrodynamics in 2+1 dimensions

abstract:
  We study the phase diagram of non compact $QED_3$ using the microcanonical
fermionic average method described elsewhere. We present evidence for a
continuous phase transition line in the $\beta, N$ plane, extending down to
arbitrarily small flavour number $N$.