The ALICE Collaboration (LHC, CERN) has studied the production of vector mesons in ultra-peripheral nuclear collisions. The VBLHEP group from JINR actively participated in these studies. This year, in September, preliminary results of measuring the excited states of ρ⁰ meson were presented at the conference QuarkMatter 2023 (Houston, USA). Now, the paper “Exclusive four pion photoproduction in ultra-peripheral Pb–Pb collisions” is under Collaboration’s review.

In ultra-peripheral collisions, the impact parameter of colliding nuclei is larger than the sum of their radii. It means that there is no strong interaction between nucleons, and particles detected by the experimental facility are the results of interaction between photons and nuclei. High intensity of the photon flux, being proportional to the square of the nucleus electric charge, ensures large cross sections for coherent photoproduction of mesons on a nucleus itself and incoherent photon scattering on a nucleon as part of a nucleus. The photon-nucleus interaction resulting in the production of a vector meson can be represented as a process of virtual fluctuation of a photon into a bound quark-antiquark state (a vector meson, since the photon spin is equal to 1), followed by elastic scattering of a meson on a nucleus through the pomeron exchange.

The STAR Collaboration (BNL) has obtained the first results on studying ultra-peripheral heavy-ion collisions at RHIC. Since the launch of the LHC, such studies have been also performed at CERN with higher energy and luminosity of the collider. “All major Collaborations at the LHC are studying ultra-peripheral collisions by the photoproduction of vector mesons and the production of muon pairs in two-photon reactions. At the same time, only the ALICE facility provides studying the photoproduction of light mesons since there is no critical experimental restrictions on the study of processes of the so called “soft” physics. A simple (only two particles from J/ψ dilepton decays or two-pion decays of ρ meson) and clear (from a topological point of view) signature of photoproduction events is not so easy for triggering these events at the facilities designed for registration of the so called “hard” physics events,” Valery Pozdnyakov, one of the members of the JINR group at ALICE, a chief researcher of the Scientific and Experimental Department of Heavy Ion Physics at LHC, VBLHEP JINR, said. He also noted that studies of the photoproduction of vector mesons can also be carried out in the planned MPD experiment (NICA).

The following is the summarised results of the analysis of the ALICE experimental data obtained with the direct and active participation of JINR employees.

Left: Invariant mass distribution fit of pion pairs; Right: Cross-section for the photoproduction of ρ⁰ in ultra-peripheral Pb–Pb collisions and corresponding theoretical calculations

The photoproduction processes of light vector mesons are of interest in terms of the possible manifestation of the black-disk limit of QCD. Previous measurements showed that using only the Gribov–Glauber approach is insufficient, and using gluon shadowing resulted in the agreement between the calculations and experimental data. For the first time, the ALICE Collaboration has measured cross-sections for the coherent photoproduction of ρ⁰ mesons in ultra-peripheral Pb–Pb collisions at √s_NN=5.02 TeV and compared it to models based on various approaches to calculations. The best agreement was obtained with calculations based on a modified vector dominance model, where the vector meson interacts with the nucleus according to the Gribov–Glauber model of nuclear shadowing. The crucial fact here is that for the first time measurements were carried out for different classes of events divided according to the presence of the electromagnetic nuclear dissociation accompanying the vector meson production. By extending the measurements described above, the first measurement of the cross-section for the coherent photoproduction of ρ⁰ mesons in ultra-peripheral Xe–Xe collisions was performed. It allowed measuring the cross-section for the photoproduction of ρ⁰ mesons as a function of the atomic number A, and the coefficient of A-dependence was measured close to 1.

The process of coherent J/ψ photoproduction on a nucleus is a good way to study gluon shadowing in a nucleus, since the cross-section of the process is expected to be proportional to the square of the gluon distribution function in a nucleus. Using large statistics collected in 2015 — 2018 runs, this cross-section was measured in ultra-peripheral Pb–Pb collisions at √s_NN=5.02 TeV. The best fit between the experimental data and theoretical models was obtained for models with moderate gluon shadowing in a nucleus. Calculations performed without talking into account the gluon shadowing, as well as the predictions from models suggesting strong gluon shadowing, differ completely from the data.

Left: Dimuon mass spectrum fit; Right: measured cross-section for the coherent J/ ψ photoproduction and theoretical calculations

The analysis of ultra-peripheral nuclear collisions in the ALICE experiment continues. “Since LHC RUN3, which continues during 2022 — 2025, the model for collecting and analyzing data at ALICE has been significantly changed – the trigger is no longer used for recording experimental data. Instead, the data are recorded in the “continuous readout” mode. As a result, the influence of the trigger efficiency, one of the factors determining the accuracy of measurements based on small multiplicity events and relatively soft energies, is leveled. Previously, it was decisive in the experiment and led to a complication of data analysis and an increase in experimental uncertainties,” explained Valery Pozdnyakov. In addition, the data analysis is performed on a platform Apache Arrow, which enables increasing data processing speed and simplifying code writing.

A group of the following authors were awarded JINR Encouraging Prize for 2022 by the results of their research on “Study of vector meson photoproduction at the ALICE experiment (CERN)”, namely Valery Pozdnyakov (VBLHEP JINR), Yulia Vertogradova (DLNP and VBLHEP JINR), Boris Rumyantsev (VBLHEP JINR), Evgeny Kryshen (PNPI NRC ‘Kurchatov Institute’), Guillermo Contreras-Nuño and David Hořák (Czech Technical University in Prague). The results are published in four papers by the ALICE Collaboration and reported eleven times at the most significant physics conferences (ICHEP, EPS, Moriond and QuarkMatter).

Publications of the series:

1. ALICE Collab., “Coherent J/ψ photoproduction at forward rapidity in ultra-peripheral Pb–Pb collisions at √sNN=5.02 TeV”, Phys. Lett. B 798 (2019) 134926;
2. ALICE Collab., “Coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb–Pb collisions at √sNN=5.02 TeV”, JHEP 06 (2020) 035;
3. ALICE Collab., “First measurement of coherent ρ0 photoproduction in ultra-peripheral Xe–Xe collisions at √sNN=5.44 TeV”, Phys. Lett. B 820 (2021) 136481;
4. ALICE Collab., “Photoproduction of low-pT J/ψ from peripheral to central Pb–Pb collisions at √sNN=5.02 TeV”, CERN-EP-2022-071, submitted to Phys. Lett.