# Ligth-flavour identified charged-hadron production in pp and Pb–Pb
collisions at the LHC

Roberto Preghenella for the ALICE Collaboration  Centro Studi e Ricerche e
Museo Storico della Fisica “Enrico Fermi”, Rome, Italy
Sezione INFN, Bologna, Italy<EMAIL_ADDRESS>
###### Abstract

Thanks to the unique detector design adopted to fulfill tracking and particle-
identification (PID) requirements (e.g. low momentum cut-off and low material
budget), the ALICE experiment provides significant information about hadron
production both in pp and Pb–Pb collisions. In particular, the $p_{\rm
T}$-differential and integrated production yields of identified particles play
a key role in the study of the collective and thermal properties of the matter
formed in high-energy heavy-ion collisions. Furthermore, the production of
high-$p_{\rm T}$ particles provides insights into the property of the hot
medium created in such collisions and the in-medium energy-loss mechanisms.

Transverse momentum spectra of $\pi^{\pm}$, K±, p and $\bar{\rm p}$ are
measured at mid-rapidity ($\left|y\right|~{}<~{}0.5$) over a wide momentum
range, from $\sim$ 100 MeV/$c$ up to $\sim$ 20 GeV/$c$. The measurements are
performed exploiting the d$E$/d$x$ in silicon and gas, the time-of-flight and
the ring-imaging Cherenkov particle-identification techniques, which will be
briefly reviewed in this report. The current results on light-flavour charged-
hadron production will be presented for pp collisions at $\sqrt{s}$ = 0.9,
2.76 and 7 TeV and for Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV.
Integrated production yields, transverse momentum spectra and particle ratios
in pp are discussed as a function of the collision energy and compared to
previous experiments and commonly-used Monte Carlo models. Pb–Pb collisions at
the LHC feature the highest radial flow ever observed and an unexpectedly low
p/$\pi$ production ratio. The results are presented as a function of collision
centrality and compared to RHIC data in Au–Au collisions at $\sqrt{s_{\rm
NN}}$ = 200 GeV and predictions from thermal and hydrodynamic models. The
nuclear modification factor ($R_{\rm AA}$) of identified hadrons will also be
discussed and compared to unidentified charged particles and theoretical
predictions. This is observed to be identical for all particle species at
high-$p_{\rm T}$.

##  1 Introduction

ALICE (A Large Ion Collider Experiment) is a general-purpose heavy-ion
detector at the CERN LHC (Large Hadron Collider). It has been designed in
order to fulfill the requirements to track and identify particles from very
low ($\sim$100 MeV/$c$) up to quite high ($\sim$100 GeV/$c$) transverse
momenta in an environment with large charged-particle multiplicities as in the
case of central lead-lead (Pb–Pb) collisions at the LHC.

;

Figure 1: Schematic layout of the ALICE detector with its main subsystems.

The ALICE experiment, shown in Figure 1, consists of a central-barrel detector
and several forward detector systems. The central system covers the mid-
rapidity region ($\left|\eta\right|\leq$ 0.9) over the full azimuthal angle.
It is installed inside a large solenoidal magnet providing a moderate magnetic
field of 0.5 T. It includes a six-layer high-resolution inner-tracking system
(ITS), a large-volume time-projection chamber (TPC) and electron and charged-
hadron identification detectors which exploit transition-radiation (TRD) and
time-of-flight (TOF) techniques, respectively. Small-area systems for
high-$p_{\rm T}$ particle-identification (HMPID), photon and neutral-meson
measurements (PHOS) and jet reconstruction (EMCal) complement the central
barrel. Thanks to these unique features the experiment is able to identify
hadrons in a wide momentum range by employing different detection systems and
techniques, as discussed in Section 2. The detectors which cover larger
rapidity regions include a single-arm muon spectrometer covering the
pseudorapidity range -4.0 $\leq\eta\leq$ -2.4 and several smaller detectors
(VZERO, TZERO, FMD, ZDC, and PMD) for triggering, multiplicity measurements
and centrality determination. A detailed description of the ALICE detector
layout and of its subsystems can be found in [1].

Since November 2009 when the first collisions at the LHC occurred, the ALICE
experiment has collected proton-proton data at several centre-of-mass energies
($\sqrt{s}$ = 0.9, 2.76, 7 and 8 TeV). During the first two LHC heavy-ion
runs, in 2010 and 2011, the detector recorded Pb–Pb collisions at a centre-of-
mass energy per nucleon pair of $\sqrt{s_{\rm NN}}$ = 2.76 TeV and could
profit from of an integrated luminosity of about 10 $\mu\rm b^{-1}$ and 100
$\mu\rm b^{-1}$, respectively. It is worth stressing the outstanding
performance of the LHC complex: the instant luminosity exceeded $10^{26}\rm
cm^{-2}s^{-1}$ in the second run, higher than the design value. Proton-lead
(p–Pb) collision data were also collected with the ALICE detector. This
occurred during a short run performed in September 2012 in preparation for the
main p–Pb run at the beginning of 2013. Eight pairs of bunches collided in the
ALICE interaction region, providing a luminosity of about $8\times 10^{25}\rm
cm^{-2}s^{-1}$. The configuration (4 TeV protons colliding with fully stripped
208Pb ions at $82\times 4$ TeV) resulted in interactions at $\sqrt{s_{\rm
NN}}$ = 5.02 TeV in the nucleon-nucleon centre-of-mass system, which moves
with a rapidity of $\Delta y_{\rm NN}$ = 0.465.

##  2 Particle identification

Figure 2: Energy loss d$E$/d$x$ in the ITS (top-left) and in the TPC (top-
right). The continuous curves represent the Bethe-Bloch parametrization.
(bottom-left) particle velocity $\beta$ measured with TOF as a function of
momentum. (bottom-right) Cherenkov angle measured in the HMPID as a function
of the track momentum. Figure 3: Transverse DCA ($DCA_{xy}$) of protons in the
range between 0.6 GeV/$c$ and 0.65 GeV/$c$ in 0-5% most central Pb–Pb
collisions together with the Monte Carlo templates which are fitted to the
data.

In this section the main particle-identification (PID) detectors relevant to
the analyses presented in this paper are briefly discussed. A detailed review
of the ALICE experiment and of its PID capabilities can be found in [2]. The
ITS is the innermost detector system, a six-layer silicon detector located at
radii between 4 and 43 cm. Four of the six layers provide specific energy loss
d$E$/d$x$ measurements and are used for particle identification in the non-
relativistic ($1/\beta^{2}$) region. By using the ITS as a standalone tracker
it is possible to reconstruct and identify low-momentum particles (below 200
MeV/c) not reaching the main tracking systems (Figure 2 top-left). The TPC is
the main central-barrel tracking detector of ALICE. It provides three-
dimensional hit information and specific energy-loss measurements with up to
159 samples. With the measured particle momentum and $\langle$d$E$/d$x\rangle$
the particle type can be determined by comparing the measurements with the
Bethe-Bloch expectation (Figure 2 top-right). The TOF detector is a large-area
array of Multigap Resistive Plate Chambers (MRPC) and covers the central
pseudorapidity region ($\left|\eta\right|<$ 0.9, full azimuth). Particle
identification is performed by matching momentum and trajectory-length
measurements performed by the tracking system with the time-of-flight
information provided by the TOF system. The overall time-of-flight resolution
is measured to be about 85 ps in Pb–Pb collisions (about 120 ps in pp
collisions) and it is determined by the time resolution of the detector itself
and by the start-time resolution (Figure 2 bottom-left). The HMPID detector
consists of seven identical proximity focusing RICH (Ring Imaging Cherenkov)
counters. Photon detection is performed using the proportional multiwire
chambers coupled to pad-segmented CsI photocathode. Particle identification is
obtained by means of the measurement of the Cherenkov angle allowing the
separation of pions and kaons between 1 GeV/$c$ and 3 GeV/$c$ and protons from
1.5 GeV/$c$ up to 5 GeV/$c$ (Figure 2).

The transverse momentum spectra of primary $\rm\pi^{\pm}$, $\rm K^{\pm}$, $\rm
p$ and $\rm\bar{p}$ are measured at mid-rapidity ($\left|y\right|~{}<~{}0.5$)
combining the techniques and detectors described above. Primary particles are
defined as prompt particles produced in the collision and all decay daughters,
except products from weak decays of strange particles. The contribution from
the feed-down of weakly-decaying particles to $\rm\pi^{\pm}$, $\rm p$ and
$\rm\bar{p}$ and from protons emitted from secondary interactions with
material are subtracted by fitting the data using Monte Carlo templates of the
DCA111Distance of Closest Approach to the reconstructed primary vertex.
distributions (Figure 3). Particles can also be identified in ALICE via their
characteristics decay topology or invariant mass. This, combined with the
direct identification of the decay daughters, allows one to reconstruct
weakly-decaying particles and hadronic resonances with a good signal-to-
background ratio.

##  3 Light-flavour hadron production

Figure 4: Transverse momentum distributions of the sum of positive and
negative pions, kaons and protons for central Pb–Pb collisions. The results
are compared to RHIC data and hydrodynamic models.

Figure 5: (left) The thermal fit of ALICE data showing data and model for the
best fit. (right) Mid-rapidity particle ratios compared to RHIC results and
predictions from thermal models for central Pb–Pb collisions at the LHC.

Figure 6: $\rm K/\pi$ (top) and $\rm p/\pi$ production ratio in pp collisions
compared with PYTHIA Monte Carlo predictions and NLO calculations.

Figure 7: Charged pion, kaon and (anti)proton nuclear modification factor $\rm
R_{AA}$ as a function of $p_{\rm T}$ for Pb–Pb collisions at $\sqrt{s_{\rm
NN}}$ = 2.76 TeV for centrality 0-5% (left) and 20-40% (right). Statistical
(vertical error bars) and systematic (gray and colored boxes) are shown for
the charged pion RAA. The gray boxes contains the common systematic error
related to pp normalization to INEL and $\rm N_{coll}$. Figure 8: Nuclear
modification factor $\rm R_{AA}$ of charged pions compared to the $\rm R_{AA}$
of unidentified charged particles as a function of $p_{\rm T}$ for different
centrality classes. Statistical (vertical error bars) and systematic (gray and
colored boxes) errors are shown for the charged pions. The colored boxes
contain the common systematic uncertainty related to the number of binary
collisions and the pp normalization to INEL. Only statistical errors are shown
for the unidentified charged $\rm R_{AA}$. Figure 9: Proton-to-pion ratio in
Pb–Pb collisions compared with pp results at 2.76 TeV in several centrality
bins. In Pb–Pb the statistical and systematic uncertainties are displayed as
error bars and gray bands, respectively. In pp the systematic uncertainty is
displayed in black squares.

Figure 10: (left) Proton-to-pion ratio in Pb–Pb collisions in several
centrality bins compared with pp results at 2.76 TeV. (right) Proton-to-pion
ratio in central Pb–Pb collisions compared with models.

ALICE has measured the production yields of primary charged pions, kaon and
(anti)protons in a wide momentum range and in several colliding systems. The
measurements have been performed in proton-proton collisions at several
centre-of-mass energies ($\sqrt{s}$ = 0.9, 2.76 and 7 TeV) and in Pb–Pb
collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV as a function of collision
centrality.

In Pb–Pb collisions the transverse momentum $p_{\rm T}$ distributions and
yields are compared to previous results at RHIC and expectations from
hydrodynamic and thermal model. The results obtained for central Pb–Pb
collisions are shown in Figure 4 and 5. The spectral shapes are harder than
those observed at RHIC, indicating an increase of the radial flow velocity
with the centre-of-mass energy. The radial flow at the LHC is found to be
about 10% higher than at RHIC energy. The hydrodynamic models shown in Figure
4 give for central collisions a fair description of the data, describing the
experimental spectra within ∼20%. This supports a hydrodynamic interpretation
of the transverse momentum distribution in central collisions at the LHC.
Further details of this anaysis and the models used to compare with the data
can be found in [5].

While the K/$\pi$ integrated production ratio is observed to be in line with
lower energy measurements and predictions from the thermal model, both the
p/$\pi$ and the $\Lambda/\pi$ ratios are lower than those at RHIC and
significantly lower (a factor $\sim$ 1.5 and 1.35, respectively) than
predictions. A possible explanation of these deviations from the thermal-model
predictions may be re-interactions in the hadronic phase due to large cross
sections for antibaryon-baryon annihilation [6, 7, 8].

The $p_{\rm T}$-dependent yield of charged kaons and protons normalized to
charged pions are shown in Figure 6 for pp collisions at $\sqrt{s}$ = 2.76 and
7 TeV where they are compared with theoretical model predictions. Within the
experimental uncertainties no energy dependence is observed in the data. The
observed production ratios are not reproduced by NLO calculations [9]. PYTHIA
Monte Carlo generator [10] underpredicts the proton-to-pion ratio at
intermediate $p_{\rm T}$.

Pion, kaon and (anti)proton production in Pb–Pb collisions have been compared
to that in pp collisions and all show a suppression pattern which is similar
to that of inclusive charged hadrons at high momenta ($p_{\rm T}$ above
$\simeq$ 10 GeV/$c$), as shown in Figure 7 and 8 [11]. This seems to suggest
that the dense medium formed in Pb–Pb collisions does not affect the
fragmentation. A similar conclusion can be drawn from the proton-to-pion ratio
measured in Pb–Pb collisions (Figure 9 and 10): for intermediate momenta (3–7
GeV/$c$) it exibits a relatively strong enhancement, by a factor of 3 compared
to that in pp collisions at $p_{\rm T}\approx$ 3 GeV/$c$ and returns to the
value in pp collisions at higher momenta ($p_{\rm T}$ above $\simeq$ 10
GeV/$c$) [11]. A similar observation was also reported for the
$\rm\Lambda/K^{0}_{s}$ ratio and possible explanations have been proposed
which include particle production via quark recombination [12].

##  4 Transverse momentum distribution in proton-lead collisions

Particle production in proton-lead (p–Pb) collisions allows one to study and
understand QCD at low parton fractional momentum $x$ and high gluon density.
It is moreover expected to be sensitive to nuclear effects in the initial
state. For this reason p–Pb measurements provide an essential reference tool
to discriminate between initial and final state effects and they are crucial
for the studies and the understanding of deconfined matter created in nucleus-
nucleus collisions.

Figure 11: The nuclear modification factor of charged particles as a function
of transverse momentum in NSD p–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02
TeV compared to measurements in central (0-5%) and peripheral (70-80%) Pb–Pb
collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV.

The measurement of the transverse momentum $p_{\rm T}$ distributions of
charged particle in p–Pb collisions were reported already in [13]. It was
previously shown that the production of charged hadrons in central Pb–Pb
collisions at the LHC is strongly suppressed [14, 15]. The suppression remains
substantial up to 100 GeV/$c$ and is also seen in reconstructed jets [16].
Proton-lead collisions provide a control experiment to clearly establish
whether the initial state of the colliding nuclei plays a role in the observed
high-$p_{\rm T}$ hadron production in Pb–Pb collisions. In order to quantify
nuclear effects, the $p_{\rm T}$-differential yield relative to the proton-
proton reference, the so-called nuclear modification factor, is calculated.
The nuclear modification factor is expected to be unity for hard processes
which exhibit binary collision scaling. This has been recently confirmed in
Pb–Pb collisions at the LHC by the measurements of direct photon [17], Z0 [18]
and W± [19], observables which are not affected by hot QCD matter. In Figure
11 the measurement of the nuclear modification factor in p–Pb collisions RpPb
is compared to that in central (0-5% centrality) and peripheral (70–80%) Pb–Pb
collisions RPbPb. RpPb is observed to be consistent with unity for transverse
momenta higher than about 2 GeV/$c$. This important measurement demonstrates
that the strong suppression observed in central Pb–Pb collisions at the LHC is
not due to an initial-state effect, but it is a final state effect related to
the hot matter created in high-energy heavy-ion reactions.

##  5 Summary and conclusions

ALICE has obtained so far a wealth of physics results both from the analysis
of proton-proton collision data and from the first two LHC heavy-ion runs.

The transverse momentum spectra of $\pi^{\pm}$, $K^{\pm}$, $p$ and $\bar{p}$
have been measured with ALICE in several colliding systems and energies at the
LHC demonstrating the excellent PID capabilities of the experiment. Data in pp
collisions show no evident $\sqrt{s}$ dependence in hadron production ratios.
In Pb–Pb collisions $\bar{p}/\pi^{-}$ integrated ratio is significantly lower
than statistical model predictions with a chemical freeze-out temperature
$T_{ch}\simeq 160-170$ MeV. The average transverse momenta and the transverse
momentum spectra indicate a $\sim$10% stronger radial flow than at RHIC
energies.

In the intermediate transverse momentum $p_{\rm T}$ region, an enhancement of
the baryon-to-meson ratio is observed. The maximum of the ratio is shifted to
higher $p_{\rm T}$ with respect to RHIC measurements. The results of the
measurements of charged pion, kaon, protons and antiproton production at high
$p_{\rm T}$ were also presented. The nuclear modification factors for these
species are similar in magnitude, suggesting that the medium does not
significantly affect fragmentation.

First results from a short pilot run with proton-lead beams have been also
reported. The measurement of the charged-particle transverse momentum $p_{\rm
T}$ spectra and nuclear modification factor in p–Pb collisions at
$\sqrt{s_{\rm NN}}$ = 5.02 TeV, covering 0.5 $<p_{\rm T}<$ 20 GeV/$c$, show a
nuclear modification factor consistent with unity for $p_{\rm T}>$ 2 GeV/c.
This measurement indicates that the strong suppression of hadron production at
high $p_{\rm T}$ observed at the LHC in Pb–Pb collisions is not due to an
initial-state effect, but is the fingerprint of jet quenching in hot QCD
matter.

## References

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