Recent results on ultra-peripheral collision studies
with ALICE at the LHC
Valery Pozdnyakov for the ALICE Collaboration
Joint Institute for Nuclear Research, Dubna, Russia

Outline:– an introduction to the physics of ultra-peripheral collisions (UPC);– J/ photoproduction in p–Pb collisions;– coherent production of J/ at forward rapidity in Pb–Pb UPC;
– coherent J/ψ and ψ′ photoproduction at midrapidity in Pb–Pb UPC;
– |t|-dependence of midrapidity coherent J/ψ photonuclear production;– coherent  production in Pb–Pb and Xe–Xe UPC with LHC Run 2 data;– summary and outlook.

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2UPC of heavy ions
Ultra-peripheral collisions (UPC)

two-photon

photonuclear

The LHC in heavy-ion mode →powerful source of quasi-real photons with intensity ~Z2.
Photon→a vector meson (VM) →scatter off a target
either coherently off whole nucleus (VM pT ~30 MeV/c)or incoherently off nucleons (VM pT~300 MeV/c).
NB there is bidirectional photon ambiguity in case of heavy ions
Large Z →huge photon fluxes →UPC can be accompanied by another photon exchange →EM nuclei excitation →neutron emission detected in Zero Degree Calorimeters.

UPC studies address gluon shadowing in nuclei in photoproduction of vector mesons,two-photon processes like light-by-light scattering, dilepton production etc.
UPC review and current status:
A.J. Baltz et al., Phys.Rept. 458 (2008) 1;
V. Guzey et al., Eur.Phys.J. C74 (2014) 7;
L. Frankfurt et al., Phys.Lett.B 752 (2016) 51; E. Kryshen, EPJ Web Conf. 204 (2019) 01011;
CMS Collab., Phys.Lett.B 797 (2019) 134826; ALICE Collab., Phys.Lett. B798 (2019) 134926;
S. R. Klein and P. Steinberg, arXiv:2005.01872 [hep-ph] (2020)
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Coherent J/ photoproduction in UPC
Quarkonium photoproduction (γA → J/ψ A) at LHC
probes high Wγp (small x) range.
The leading order cross section(assuming that gluons have ~ same x, i.e. x1 ≈ x2)

M. G. Ryskin, Z. Phys. C57 (1993), 89

extensions of the calculations to NLO are discussed inFlett CA, et al. arXiv:1912.09128 [hep-ph] (2019)
Phys.Rev. C85 (2012) 044904

how to properly incorporate nuclear shadowing
at small x?

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A Large Ion Collider Experiment (ALICE) at LHC

ALICE systems relevant for J/photoproduction measurements:- Muon spectrometer (item 12 on scheme) /TPC (3) to reconstruct J/ or 0 decays;- Trigger detectors: ITS SPD (1), V0 (2), AD (17), TOF (5) and muon trigger chambers (14);- Zero Degree Calorimeters (18) to detect neutrons from nucleus EM dissociation.
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ALICE event displays
forward dimuonspair of charged particles
in barrel

Main features of UPC vector meson photoproduction:- exclusive events, only vector meson decay particles detected;- transverse momentum balance of final state particles.

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J/ photoproduction in p-Pb collisions
p-Pb collisions have
– no photon source uncertainty since photon flux comes almost entirely from Pb-side
and gamma-proton centre-of-mass energy is calculated w/o ambiguity;– background from gamma-gamma to lepton pair small;– inelastic interactions effectively suppressed by no-ZDC signal requirement.
– Wp [20 – 700] GeV;
– x [10−5 – 10−2]– Run 2 data should reach W beyond 1 TeV
CCT (J. Cepila, J.G. Contreras, J.D. Tapia Takaki,
Phys. Lett. B 766, 186 (2017)):colour dipole approach + energy dependence ofgeometrical fluctuations of the proton structure in theimpact parameter plane.
JMRT (S.P. Jones, A.D. Martin, M. Ryskin, T. Teubner,
JHEP 11, 085 (2013)):the parameters for calculations obtained by a fit to thedata and their energy dependence.

Eur. Phys. J. C79 (2019) 402

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CGC (N. Armesto, A.H. Rezaeian,Eur. Phys.Phys.
J. C79402
Rev.(2019)
D 90(5),054003 (2014))
colour dipole model.
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7Сoherent production of J/ at forward rapidity in Pb–Pb UPC
Increased LHC Run 2 Pb-Pb luminosity (∫L > 700 μb−1), larger J/ production cross section
and more efficient event triggering provides ~200 times larger J/ yield as compared to the Run 1 data.
Forward ALICE Diffractive (AD) detector installed for Run 2 and included into the trigger suppresses
background from peripheral heavy-ion collisions.

Run 2

Run 2
Run 1

Phys.Lett.B718 (2013) 1273

Phys.Lett. B798 (2019) 134926
Phys.Lett. B798 (2019) 134926
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8Сoherent production of J/ at forward rapidity in Pb–Pb UPC (cont'd)

Phys.Lett. B798 (2019) 134926
impulse approximation: no nuclear effects
STARLIGHT: VDM + Glauber
Comput. Phys. Commun. 212 (2017) 258
EPS09 LO (GKZ) / LTA (GKZ)
PRC93 (2016) 055206
GM: color dipole + IIM / bCGC CGC
PRC 90 (2014) 015203, JPG 42 (2015) 105001
Ipsat (LM): color dipole + IPSat CGC
PRC 83 (2011) 065202, PRC 87 (2013) 032201
GG-HS (CCK): color dipole + energy dependent hot-spot model PL B766 (2017) 186, PRC 97 (2018), 024901
BGK-I (LS): color dipole + CGC
Phys. Rev. C 99, 044905 (2019)

Coherent J/ photoproduction cross section implies moderate gluon shadowing in nuclei.
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9Coherent J/ψ and ψ′ photoproduction at midrapidity in Pb-Pb UPC
J/→   J/→e+e-arXiv:2101.04577

arXiv:2101.04577

S→e+e-    S→        

arXiv:2101.04577
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arXiv:2101.04577
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10Coherent J/ψ and ψ′ photoproduction at midrapidity in Pb-Pb UPC (cont'd)
J/
S

arXiv:2101.04577

arXiv:2101.04577(models described on slide 8)

The nuclear gluon shadowing factor is found to be ~0.65 at x values ~10−3.
Few models (based on leading twist approximation of nuclear shadowing, on the EPS09
parameterization or the energy-dependent hot-spot model) describe the measured coherent
J/ψ cross sections at forward and in central rapidities with a tension at semi-forward rapidity.
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|t|-dependence of midrapidity coherent J/ψ photonuclear production
The dependence on |t|, the square of the momentum transferred between the incoming andoutgoing target nucleus, of coherent J/ψ photoproduction at Bjorken-x range ~10−3 was measured
thus provided an info to the transverse gluon nPDF.

Phys.Lett. B817 (2021) 136280

Poor description by STARlight model used only Pb nuclear form factor.
Better agreement with models with shadowing by the leading-twist approximation
or gluon-saturation effects from the impact-parameter dependent Balitsky–Kovchegov approach.
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12Сoherent  production in Pb–Pb UPC with LHC Run 2 data
Photonuclear production of ρ0 in Pb–Pb UPC at the LHC has a large cross section
which makes it a good tool to study the approach to the black-disk limit of QCD.
The integrated luminosity is of 485 mb−1.
The signal sample contains ~57 thousand events
which passed selection:– |rapidity of a pair| < 0.8;
– pair transverse momentum pT < 0.2 GeV/c;– 0.55 < pair mass < 1.4 GeV/c2.
overall experimental efficiency of ~17%

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Fit byBreit-Wigner for  resonance shape+ constant term for direct pion pair production+ interference between them
+ dimuons from gamma-gamma interactions.

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13Сoherent  production in Pb–Pb UPC with LHC Run 2 data (cont'd)Model predictions:
GKZ (V. Guzey, E. Kryshen and M. Zhalov,Phys. Rev. C93 (2016) 055206 ):VDM + Gribov-Glauber model of nuclear shadowing
for fluctuations of the photon-nucleons interaction.
CCKT (J. Cepila, J. G. Contreras, M. Krelina, andJ.D. Tapia Takaki,
Nucl. Phys. B934 (2018) 330–340):
colour-dipole model + gluons “hot spots” of thestructure of the nucleon in the transverse plane +Glauber model for nuclear effects.
GMMNS (Goncalves, Machado, Morerira, Navarra
and dos Santos,Phys. Rev. D96 (2017) 094027 ):Iancu-Itakura-Munier (IIM) approach for gluon
saturation + colour-dipole model.
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STARLIGHT (S.Klein, J.Nystrand et al.
Comp.Phys.Comm. 212 (2017) 258) :+p → VM+p cross section + the optical theorem +a Glauber-like eikonal formalism

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14Сoherent  production in Pb–Pb UPC with LHC Run 2 data (cont'd)
ALICE is equipped with two ZDC detectors (ZNA and ZNC)
– located at either side of interaction point at ±112.5 m along the z-axis;– intended for measurement of neutrons at beam rapidity;– which provide time resolution enough to separate beam–beam and beam–gas interactions;– which have a good efficiency to detect neutrons coming from electromagnetic dissociation(EMD) with |η | > 8.8;
– which have a relative energy resolution of around 20% for a neutron which allows toseparate events with either zero or a few neutrons at beam rapidities.

0nxn
xnxn
0n1n0n0n
xn0n2n3n
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15Сoherent  production in Pb–Pb UPC with LHC Run 2 data (cont'd)
0nxn+ xn0n0n0nJHEP 06 (2020) 35

xnxn

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agreement both with models based on colour-dipole
approach and with Gribov-Glauber shadowing.
The models for EMD accompanying VM
photoproduction describe the measured crosssections for different neutron emission classes which
are sensitive to different impact parameter ranges.

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16Сoherent  production in Xe–Xe UPC with LHC Run 2
Shadowing effects are expected to depend on the atomic number of the nucleus→ UPC measurements for different A extend the study of the shadowing
The LHC collided xenon nuclei for the first time with a short accelerator run in fall of 2017.
The experimental conditions for UPC triggering and reconstruction stay close to Pb runs.
The luminosity is of (279.5 ± 29.9) mb−1.
The events which passed selection:– |pair rapidity| < 0.8;
– pair pT < 0.15 GeV/c;– pair mass > 0.55 GeV/c2.
Around 1.8 thousand events were identified
experimental efficiency of ~15%.

Phys.Lett. B820 (2021) 136481

The measured photoproduction cross section
131.5 ± 5.6 (stat.) +17.5−16.9 (syst.) mb
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Fit done similar to Pb-Pb analysis by
Breit-Wigner for resonance shape+ constant term for direct pair production+ interference between them+ dimuons from  interactions.
Overall systematic uncertainty ~13%17Сoherent  production in Xe–Xe UPC with LHC Run 2 (cont’d)range

Phys.Lett. B820 (2021) 136481

The models are close to each other at zero-rapidity andoverestimate the data.
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GKZ (V. Guzey, E. Kryshen andM. Zhalov, Phys. Lett. B782 (2018) 251):
VDM+ Gribov-Glauber model of nuclear
shadowing for fluctuations of thephoton-nucleons interaction;
CCKT (J. Cepila, J. G. Contreras, M.
Krelina, and J.Tapia Takaki, Nucl. Phys.
B934 (2018) 330–340):
colour-dipole model + gluons “hot spots”
of the structure of the nucleon in thetransverse plane +Glauber model for
nuclear effects;
GMMNS (Goncalves, Machado, Morerira,Navarra and dos Santos, Phys. Rev. D96(2017) 094027 ):Iancu-Itakura-Munier (IIM) approach forgluon saturation + colour-dipole model;
STARLIGHT (S.Klein, J.Nystrand et al.
Comp. Phys. Comm. 212 (2017) 258) :+p → VM+p cross section + the opticaltheorem + Glauber-like eikonal formalism.
18Сoherent production in UPC

The coherent ρ0 photoproduction
cross section at zero-rapidity forPb–Pb and for Xe–Xe converted
into A measurement by using thephoton fluxes of58.6 (for Xe) and
128.1 (for Pb).
The A-dependence is fitted by apower-law model, slope parameter
is found to be:
0.963±0.019 (ALICE+H1)
0.985±0.007 (GKZ model)
0.983±0.004 (CCKT model)
Phys.Lett. B820 (2021) 136481

Both slopes of GKZ and CCKT models are in good agreement with that found in the fit of the data.
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Conclusions and outlook– analysis of J/ and ρ0 photoproduction is a tool to learn about the dynamics of Ainteractions and the gluon content of nuclei;– exclusive photoproduction of J/ψ off proton measured in p–Pb UPC in the γp massinterval 40 < Wγp < 550 GeV;– cross sections of coherent forward and midrapidity J/ photoproduction were measured inPb–Pb UPC collisions at √sNN = 5.02 TeV. The result implies moderate gluon shadowing
in nuclei;– ' production at mid-rapidity in Pb–Pb UPC was studied;– rapidity dependence of the coherent ρ0 production cross section in Pb–Pb UPC
at √sNN = 5.02 TeV was measured for different cases of EMD accompanying VM
photoproduction. The cross sections are compared with the main available models;– for the first time the cross section of coherent production in Xe–Xe UPC is measured.
The A dependence of A interactions is fitted using Pb–Pb, Xe–Xe UPC and H1 data as well.
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