$\ast$$\ast$footnotetext: Speaker

# New measurements on diffractive vector mesons

Simone Ragonia,∗, for the ALICE and LHCb collaborations

###### Abstract

Vector meson photoproduction at high energies has attracted increased interest
in recent years due to the unique kinematical ranges offered by the LHC
experiments. The ALICE and LHCb collaborations have provided measurements of
vector meson photoproduction on nuclear and proton targets, i.e. off p, Pb and
Xe targets. The large data sets available from LHC Run 2 allow for more
differential measurements.

The ALICE and LHCb collaborations have recently provided measurements of
coherent ${\mathrm{J}}/\psi$ in Pb–Pb ultra-peripheral collisions (UPC). In
addition, the former also reports the first measurement of the
$|t|$-dependence of coherent ${\mathrm{J}}/\psi$ in Pb–Pb, and has also
recently measured coherent $\rho^{0}$ photoproduction off Pb and Xe targets,
thus providing the first study of the A dependence of coherent $\rho^{0}$
photoproduction.

Vector meson photoproduction in peripheral Pb–Pb collisions has also been
observed by ALICE and LHCb, thus providing a viable technique to extract
photonuclear cross sections at a low Bjorken-$x$ of around $10^{-5}$. Lastly,
the potential of UPC for tetraquark searches is briefly discussed.

##  1 Introduction

The LHC experiments offer new and hitherto unexplored kinematical ranges for
the investigation of vector meson photoproduction, thus renewing the interest
in these processes. The ALICE, ATLAS, CMS, and LHCb collaborations all report
measurements for ultra-peripheral collisions, and this contribution will focus
on the results on vector meson photoproduction from ALICE and LHCb. The main
motivation behind this interest, lies in the possibility of studying the gluon
distributions at low Bjorken-$x$. The Bjorken-$x$ involved in the process can
be directly related to the rapidity of the vector meson in the final state,
via $x=\frac{M_{\rm VM}}{\sqrt{s_{\rm NN}}}\cdot\exp(\pm|y|)$ [1]. It is then
possible to probe the gluon PDFs down to $x{\sim}10^{-6}$ in pp at LHCb [2, 3,
4], with the coverage in $W_{\gamma{\rm p}}$ available to ALICE and LHCb, as
shown in Fig. 1 for exclusive ${\mathrm{J}}/\psi$ photoproduction [4]. Both
sets of data roughly follow a power law, as previously observed with H1 data
[5]. However, at higher energies, the LHCb high $W_{\gamma{\rm p}}$ solutions
agree better with the NLO JMRT prediction, which deviates somewhat from power
law.

Figure 1:  Exclusive ${\mathrm{J}}/\psi$ production cross section as a
function of the centre-of-mass energy of the $\gamma$p system [4, 6, 7, 8].

##  2 Coherent ${\mathrm{J}}/\psi$ photoproduction in Pb–Pb collisions

Coherent ${\mathrm{J}}/\psi$ photoproduction has been measured in Pb–Pb ultra-
peripheral collisions (UPC) by the ALICE and LHCb collaborations [9, 10, 11].
In UPCs, the two Pb ions are at large impact parameters $b$, greater than the
sum of the two nuclear radii, i.e. $b>2\cdot R_{\rm Pb}$, where $R_{\rm Pb}$
is the radius of the Pb ion. A possible process is when a photon from one
nucleus interacts coherently with the other nucleus, resulting in a vector
meson in the final state. The nuclei remain largely intact. The final state is
thus especially clean: only the decay products of the vector meson are
observed, in an otherwise empty detector. The main purpose of coherent
${\mathrm{J}}/\psi$ photoproduction is to deepen understanding of nuclear
shadowing. The nuclear suppression factor, $S_{{\rm Pb}(x)}$, is an
interesting variable to gauge the nuclear shadowing involved in a measurement
[12]. It is defined starting from the measured cross section, as shown in the
following Eq. 1 [12]:

$S_{{\rm Pb}(x)}=\sqrt{\frac{\sigma(\gamma A\longrightarrow{\mathrm{J}}/\psi
A)_{measured}}{\sigma(\gamma A\longrightarrow{\mathrm{J}}/\psi A)_{\rm
IA}}}\text{ ,}$ (1)

where $\sigma(\gamma A\longrightarrow{\mathrm{J}}/\psi A)_{measured}$ is the
measured cross section for coherent ${\mathrm{J}}/\psi$ photoproduction, while
$\sigma(\gamma A\longrightarrow{\mathrm{J}}/\psi A)_{\rm IA}$ is the cross
section computed for the Impulse Approximation model, which neglects nuclear
effects. The ALICE and LHCb measured cross sections agree within current
systematical uncertainties, as shown in Fig. 2 [12, 9, 10, 11]. The cross
sections can be simultaneously fitted to measure the nuclear suppression
factor. The data are consistent with $S_{{\rm Pb}(x)}\sim 0.6$ with a 5%
accuracy at $x=6\cdot 10^{-4}-10^{-3}$[12, 10]. The statistical and systematic
uncertainties from the measured cross sections are added in quadrature
together for the fit.

Figure 2:  Fit to the ALICE and LHCb measurements for coherent
${\mathrm{J}}/\psi$ photoproduction in Pb–Pb UPCs [12]. The ALICE and LHCb
cross sections agree within current systematical uncertainties. The fit
favours a $S_{{\rm Pb}(x)}\sim 0.6$ with a 5% accuracy at $x=6\cdot
10^{-4}-10^{-3}$.

Ultimately, the small uncertainties on $S_{{\rm Pb}(x)}$ demonstrate the
potential of UPC measurements for low-$x$ constraints. Already they resulted
in a significant reduction of the uncertainties on EPPS16 nuclear PDFs [12].

##  3 Extracting photonuclear cross sections in nucleus–nucleus collisions

Vector meson photoproduction in A–A collisions suffers from an intrinsic
ambiguity, due to the process being a superposition of two diagrams, i.e. the
system is symmetric. This implies that the cross section
${\mathrm{d}}\sigma_{\rm PbPb}/{\mathrm{d}}y$ has two contributions, as shown
in Eq. 2:

${\mathrm{d}}\sigma_{\rm
PbPb}/{\mathrm{d}}y=n(\gamma,+|y|)\cdot\sigma_{\gamma{\rm
Pb}}(+|y|)+n(\gamma,-|y|)\cdot\sigma_{\gamma{\rm Pb}}(-|y|)\text{ ,}$ (2)

where $n(\gamma,\pm|y|)$ represents the photon flux as a function of rapidity,
and $\sigma_{\gamma{\rm Pb}}(\pm|y|)$ the photonuclear cross section. It is
possible to disentangle the two photonuclear cross sections by measuring cross
sections at different impact parameters. Two techniques have been considered,
namely peripheral photoproduction [13], and photoproduction accompanied by
neutron emission [14]. The strategy for the latter is to tag an event based on
the neutron activity seen by Zero Degree Calorimeters. UPC events accompanied
by neutron emission are characterised by smaller average impact parameters
[15]. Peripheral collisions are characterised by an impact parameter $b<2\cdot
R_{\rm Pb}$. The ALICE [16], STAR [17], and LHCb collaborations [18] have
observed coherent ${\mathrm{J}}/\psi$ photoproduction in peripheral
collisions.

##  4 Coherent $\rho^{0}$ and A dependence

The ALICE Collaboration has measured coherent $\rho^{0}$ photoproduction in
Pb–Pb [19] and Xe–Xe collisions [20]. It is thus now possible to perform the
first fit to the available measured cross sections for $\gamma$p, $\gamma$Pb,
and $\gamma$Xe, to measure the A dependence of the coherent $\rho^{0}$ cross
section. This is shown in Fig. 3, where the fit is performed. A power-law
model of the form $\sigma_{\gamma{\rm A}}=\sigma_{0}\cdot{\rm A}^{\alpha}$, is
used to model the trend. The results are compared to three extreme scenarios
for the slope $\alpha$: 4/3, 1, and 2/3, for coherent, incoherent and black-
disk limit, respectively. The slope is found to be quite close to 1, which
does not signify incoherent behaviour but it is due to large nuclear shadowing
suppression [20].

Figure 3: Left: fit to the available coherent $\rho^{0}$ photoproduction
measurements. Nuclear effects are found to be important [20]. Right: effective
mass distribution for ${\mathrm{J}}/\psi$ ${\mathrm{J}}/\psi$ pairs in central
exclusive production (CEP) events, i.e. pp $\longrightarrow$pp
$+X\longrightarrow$pp $+{\mathrm{J}}/\psi+{\mathrm{J}}/\psi$ with the LHCb
detector [21].

##  5 Tetraquark searches and UPC

The LHCb Collaboration has recently observed a structure in the
di-${\mathrm{J}}/\psi$ invariant mass spectrum with inclusive pp data,
$X(6900)$ [22]. This resonance could originate from a tetraquark state with
four charm quarks, i.e. $T_{cc\bar{c}\bar{c}}$, although further
investigations are required to ultimately confirm the nature of the resonance.

It was recently argued [23] that UPCs might prove to be an interesting
opportunity to investigate $T_{QQ\bar{Q}\bar{Q}}$ states, where $Q=c,b$. The
processes $\gamma\gamma\longrightarrow{\mathrm{J}}/\psi{\mathrm{J}}/\psi$ and
$\gamma\gamma\longrightarrow\Upsilon\Upsilon$ could provide further insights
for possible tetraquark states. The LHCb Collaboration had observed the
production of ${\mathrm{J}}/\psi$ pairs in exclusive processes, i.e. pp
$\longrightarrow$pp $+X\longrightarrow$pp
$+{\mathrm{J}}/\psi+{\mathrm{J}}/\psi$ with LHC Run 1 data [21]. As shown in
Fig. 3, the few observed candidates lie in the interesting invariant mass
region of the $X(6900)$ resonance.

##  6 Conclusions

Vector meson photoproduction at LHC energies offers the possibility of
addressing gluon distributions at low Bjorken-$x$ down to $x\sim 10^{-6}$. The
available measurements on coherent ${\mathrm{J}}/\psi$ photoproduction better
constrain the uncertainties on the nuclear suppression factor. The first fit
to the available measurements of coherent $\rho^{0}$ photoproduction finds
significant nuclear effects. Finally, UPCs offer new opportunities for
tetraquark investigations with the ALICE and LHCb detectors.

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