Proceedings of the International Workshop on the Physics of Ultra Peripheral Collisions
Proceedings of the International Workshop on the Physics of Ultra-Peripheral Collisions (Phys. Proc. UPC) serves as the central platform for the proceedings of the International Workshop on the Physics of Ultra-Peripheral Collisions. Proceedings of the International Workshop on the Physics of Ultra Peripheral Collisions will be published biannually. Topics include: Recent experimental results at HERA, RHIC and LHC Theoretical description of exclusive photon-proton and photon-nucleus scattering Nuclear PDFs developments using photon-induced data Nonlinear and gluon saturation effects in photon-nucleus scattering Hadronization in exclusive processes Soft nucleon and nucleus interactions Photoproduction in events with nuclear overlap UPCs and future electron-ion colliders
Recent results from ultra-peripheral lead-lead collisions with ATLAS
Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This report presents a series of measurements of such processes performed by the ATLAS Collaboration. Measurements of exclusive dilepton production (electron, muon, and tau pairs) are discussed. These processes provide strong constraints on the nuclear photon flux and its dependence on the impact parameter and photon energy. In particular, measurements of the cross-sections in the presence of forward neutrons provide an additional experimental handle on the impact parameter range sampled in the observed events. Furthermore, the tau-pair production measurements can constrain the anomalous magnetic dipole moment of the tau lepton. High statistics measurements of light-by-light scattering shown in this talk provide a precise and unique opportunity to investigate extensions of the Standard Model, such as the presence of axion-like particles. The measurement of charged hadron in photonuclear collisions using 5.02 TeV Pb+Pb data collected in 2018 by ATLAS is also discussed. The charged hadron particle yields are presented as a functions of pseudorapidity and transverse momentum in different categories of event multiplicity. Measurement of jet production in ultra-peripheral collisions performed with the ATLAS detector using high-statistics 2018 Pb+Pb data is presented.
Copyright (c) 2024 K. Maj, on behalf of the ATLAS Collaboration https://creativecommons.org/licenses/by/4.0
We present a new extension in Pythia Monte Carlo event generator that allows to simulate the leading contribution in collisions of a real photon and a heavy ion. The model is based on a vector meson dominance (VMD) where a real photon is modelled as a linear combination of different vector-meson states, and an extension of Angantyr, the heavy-ion model in Pythia, that allow simulations of a generic hadron colliding with an ion target. We first verify the VMD implementation by comparing simulations to HERA photoproduction data. Then we present simulated results corresponding to event selection criteria applied in a recent ATLAS analysis for ultra-peripheral Pb-Pb collisions at the LHC. We find that the simulated results are in line with the ATLAS data when accounting for the limited detector efficiency for charged-particle reconstruction. We also consider two-particle correlations and study whether the simulated events reproduce the collective behaviour seen in the ATLAS photon-lead data.
Copyright (c) 2024 https://creativecommons.org/licenses/by/4.0
Exclusive η_c production by γ*γ interactions in electron-ion collisions
One of the main goals of future electron-ion colliders is to improve our understanding of the structure of hadrons. We study the exclusive ηc production by γγ* interactions in eA collisions and demonstrate that future experimental analysis of this process can be used to improve the description of the transition form factor. The rapidity, transverse momentum, and photon virtuality distributions are estimated considering the energy and target configurations expected to be present at the EIC, EicC, and LHeC and assuming different predictions for the light-front wave function of the meson. Our results indicate that electron-ion colliders can be considered an alternative for providing supplementary data to those obtained in colliders.
Copyright (c) 2024 I. Babiarz, V. P. Goncalves, W. Schäfer, A. Szczurek https://creativecommons.org/licenses/by/4.0
The ALICE experiment has undergone a major detector upgrade for Run 3, expanding its detection capabilities for a wide variety of studies. The new continuous readout has significantly enhanced the physics potential for ultra-peripheral collision analyses. In this talk, we discussed some of the physics analyses that can be carried out in ultra-peripheral collisions using the Run 3 data and presented some of the first physics performance plots in both proton-proton and heavy-ion collisions.
Copyright (c) 2024 Anisa Khatun (for the ALICE Collaboration) https://creativecommons.org/licenses/by/4.0
Light-by-light scattering in ultraperipheral collisions of heavy ions with future FoCal and ALICE 3 detectors
I present possible future studies of light-by-light scattering using FoCal@ALICE and ALICE 3 detectors. Different mechanisms are discussed. The PbPb → PbPb γγ cross section is calculated within the equivalent photon approximation in the impact parameter space. Several differential distributions are presented and discussed. We predict the cross section in the (mb-b) range for typical ALICE 3 cuts, a few orders of magnitude larger than for the current ATLAS or CMS experiments. We also consider the two-π⁰ background, which can, in principle, be eliminated in the new kinematical range for the ALICE 3 measurements by imposing dedicated cuts on diphoton transverse momentum and/or so-called vector asymmetry.
Copyright (c) 2024 Antoni Szczurek https://creativecommons.org/licenses/by/4.0
Azimuthal angular correlations in lepton pair production in ultra-peripheral heavy ion collisions
The coherent photons induced by relativistic heavy ions are highly linearly polarized, in close analogy to the linear polarization of gluons in a large nucleus. We proposed to measure the photon polarization through azimuthal asymmetries in dilepton production in ultra-peripheral collisions. Our prediction for the asymmetries were soon confirmed by the STAR experiment with high precision. We refined our analysis recently by including the final state soft photon radiation effect beyond the double leading logarithm approximation. The azimuthal asymmetries and acoplanarity at relatively high transverse momentum provide unique opportunities to test the resummation formalism thanks to the extremely high photon flux in UPCs. Our results clearly show the feasibility to access the sub-leading resummation effects in UPCs at the RHIC and LHC.
Copyright (c) 2024 Ya-jin Zhou https://creativecommons.org/licenses/by/4.0
Novel aspects of particle production in ultra-peripheral collisions
One of the hot topics in hadron physics is the study of the new exotic charmonium states and the determination of their internal structure. Another important topic is the study of the magnetic field produced in relativistic heavy-ion collisions and its effects on observables. In this note, we show that we can use ultra-peripheral collisions to address both topics. We compute the cross section for the production of the D⁺D⁻ molecular bound state in photon-photon collisions and also the cross section for π⁰ production in the target induced by the magnetic field of the projectile. Both cross sections are sizeable, and their measurement would be very useful to elucidate the above-mentioned questions.
Copyright (c) 2024 F.S. Navarra, F.C. Sobrinho, C.A. Bertulani, I. Danhoni https://creativecommons.org/licenses/by/4.0
Photoproduction of J/ψ and dileptons in Pb–Pb collisions with nuclear overlap
Photon-photon reactions and the production of J/ψ meson through photonuclear reactions have been extensively studied in ultra-peripheral heavy-ion collisions, in which the impact parameter is larger than twice the nuclear radius. In recent years, coherently photoproduced J/ψ and dilepton production via photon-photon interactions have also been observed in nucleus-nucleus (A–A) collisions with nuclear overlap. The former can help to constrain the nuclear gluon distributions at low Bjorken-x and high energy, while the latter could be used to further map the electromagnetic fields produced in heavy-ion collisions. In addition, these measurements can shed light on the theory behind photon-induced reactions in A–A collisions with nuclear overlap, including possible interactions of the measured probes with the formed and fast expanding quark-gluon plasma. Since the produced quarkonium is expected to keep the polarization of the incoming photon due to s-channel helicity conservation, the photoproduction origin of the J/ψ yield excess at very low transverse momentum, pT, can be confirmed by the measurement of the J/ψ polarization. The ALICE detector can perform quarkonium production measurements at both mid (|y|<0.9) and forward (2.5<y<4) rapidities down to pT = 0. In the following, the new ALICE measurements of the J/ψ y-differential cross section and the first polarization results of coherently photoproduced J/ψ via the dimuon decay channel at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV are reported. Additionally, the measurement of an excess with respect to expectations from hadronic production in the dielectron yield, at low mass and pT, at midrapidity in Pb–Pb collisions at √sNN = 5.02 TeV, is presented. The results are compared with available theoretical models.
Copyright (c) 2024 N. Biz´e (for the ALICE Collaboration) https://creativecommons.org/licenses/by/4.0
Axion Physics in UPC and LbyL Scattering
In this work, we revisited the light-by-light scattering physics in ultraperipheral collisions. Furthermore, an extension model with axion-like particles is also studied. We used MadGraph5\_aMC@NLO+gamma-UPC event generator for our analysis and the proposed kinematic cuts for the ALICE 3 experiment were considered. Different distribution functions for both light-by-light and axion-like particles are analyzed and we find that the transverse momentum is a good channel to observe the production of axions in ultraperipheral collisions.
Copyright (c) 2024 C. O. Ram´ırez- ´Alvarez, A. Fern´andez-T´ellez, A. I. Hern´andez-Ju´arez https://creativecommons.org/licenses/by/4.0
Collision geometry in UPC dijet production
We present a comprehensive NLO pQCD study on inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs). Our analysis takes into account the finite size of both the photon-emitting and the target nucleus, estimated using the Wood-Saxon nuclear density profile. We show that a significant part of the measured dijets at large z_\gamma in UPC Pb+Pb collisions at 5.02 TeV come from events with relatively small impact parameters of the order of a few nuclear radii, and the cross section predictions thus become sensitive to the modelling of the collision geometry and photon flux near the source nucleus. In addition, we include the modelling of electromagnetic breakup survival factor needed for a direct comparison with the experimental data and study the resolution power of this process in terms of the impact-parameter dependent nuclear parton distribution functions.
Copyright (c) 2024 Petja Paakkinen, Kari J. Eskola, Vadim Guzey, lkka Helenius, Hannu Paukkunen https://creativecommons.org/licenses/by/4.0
Inclusive and Diffractive Dijet Photoproduction in Ultraperipheral Pb-Pb Collisions at the LHC
In this contribution, we summarize NLO pQCD predictions for inclusive and diffractive dijet photoproduction in Pb-Pb UPCs at the LHC. We demonstrate that the theory describes well the preliminary ATLAS data on the inclusive cross section, which probes nuclear parton distributions (PDFs) down to x_A ~ 0.005 and which can reduce current uncertainties of the small-x nuclear gluon distribution by approximately a factor of 2. Employing predictions of the leading twist approach to nuclear shadowing for nuclear diffractive PDFs, we calculate the cross section of diffractive dijet photoproduction and show that its x_gamma} dependence is sensitive to the effect of nuclear shadowing and the mechanism of QCD factorization breaking in hard diffraction. We also find that due to large leading twist nuclear shadowing and restricted kinematics, the diffractive contribution to the inclusive cross section of dijet photoproduction does not exceed 5-10%, which helps with an ambiguous interpretation of the ATLAS data.
Copyright (c) 2024 Vadim Guzey https://creativecommons.org/licenses/by/4.0
Nuclear Shadowing in inelastic photon-nucleus scattering in UPCs with forward neutrons
We argue that measurements of forward neutrons from nuclear breakup in inclusive high energy photon-nucleus (photon-A) scattering provide a novel complementary way to study small-x dynamics of QCD in heavy-ion ultraperipheral collisions (UPCs). Using the leading twist approach to nuclear shadowing, we calculate the distribution over the number of evaporation neutrons produced in photon-lead collisions at the LHC. We demonstrate that it allows one to determine the distribution over the number of wounded nucleons (inelastic collisions), which constrains the mechanism of nuclear shadowing of nuclear parton distributions and gives an access to their impact parameter dependence.
Copyright (c) 2024 Mark Strikman, Vadim Guzey, Massimiliano Alvioli https://creativecommons.org/licenses/by/4.0
The ratio of Ψ(2S) and J/Ψ exclusive photoproduction cross-sections as a tool to detect non-linear QCD evolution
We study the proposal that the rise with energy of the ratio of Ψ(2s) and J/Ψ exclusive photoproduction cross-sections might serve as an indicator of the presence of non-linear QCD evolution, related to the presence of high and potentially saturated gluon densities in both the proton and a lead nucleus. Our study employs recent fits of the GBW and BGK dipole models and provides predictions for both exclusive photoproduction on a proton and on a lead nucleus. While the cross-sections for photoproduction on a proton depend only weakly on non-linear low x corrections, we find an increased sensitivity for the cross-section ratio, which is directly related to the node in the Ψ(2s) wave function. We further give a description of recent ALICE data for exclusive J/Ψ photoproduction on a lead nucleus and provide predictions for Ψ(2s) photoproduction on a lead nucleus as well as for the corresponding cross-section ratio.
Copyright (c) 2024 Martin Hentschinski, Marco Antonio Alcazar Peredo https://creativecommons.org/licenses/by/4.0
Two-body densities as a framework for dynamical imaging and their connection to ultra-peripheral collisions
We present results on two-parton densities in coordinate space which capture a fuller dynamical picture of the proton’s internal structure, including information on the relative position between quarks and gluons in the transverse plane. The connection of such two-body densities to observables, proceeds in QCD, via the definition of double generalized parton distributions (DGPDs) that can be accessed in the production of two vector mesons, or two dimuon systems in ultraperipheral collisions (UPCs) through a double scattering process.
Copyright (c) 2024 Zaki Panjsheeri, Joshua Bautista, Simonetta Liuti https://creativecommons.org/licenses/by/4.0
The production of four pion events in ultraperipheral heavy-ion collisions at RHIC and LHC energies is studied. Preliminary H1 data is used to enhance the understanding of the poorly known process. Predictions for photon-nucleus interactions are calculated for various excited states of mesons. Agreement between theoretical predictions and available STAR data at RHIC is presented. The comparison of the four-pion invariant mass spectrum and nuclear total cross section indicates that rho(1570) plays a crucial role in accurately describing existing experimental data. Nuclear predictions for LHC energy in the central region of rapidity are also provided.
Copyright (c) 2024 Mariola Klusek-Gawenda https://creativecommons.org/licenses/by/4.0
Measurement of dilepton production from photon fusion processes in Pb+Pb UPC with the ATLAS detector
Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This report presents a series of measurements of dilepton production from photon fusion performed by the ATLAS Collaboration. Recent measurements of exclusive dielectron production in ultra-peripheral collisions (UPC) are presented. These processes provide strong constraints on the nuclear photon flux and its dependence on the impact parameter and photon energy. Comparisons of the measured cross sections to QED predictions from the STARLight and SuperChic models are also presented. Tau-pair production measurements can constrain the tau lepton's anomalous magnetic dipole moment (g-2), and a recent ATLAS measurement using muonic decays of tau leptons in association with electrons and tracks provides one of the most stringent limits available to date. Furthermore, measurements of muon pairs produced via two-photon scattering processes in hadronic (i.e. non-UPC) Pb+Pb collisions are discussed. These non-UPC measurements provide a novel test of strong-field QED and may be a potentially sensitive electromagnetic probe of the quark-gluon plasma. These measurements include the dependence of the cross section and angular correlation on the mean transverse momenta of the dimuon pair, the rapidity separation between the muons, and the angle of the pair relative to the second-order event-plane, all measured differentially as a function of the Pb+Pb collision centrality.
Copyright (c) 2024 I. Grabowska-Bold (for the ATLAS Collaboration) https://creativecommons.org/licenses/by/4.0
Impact parameter dependence of dilepton production: Wigner function approach and the role of photon polarizations
We revisit the Wigner function approach to the impact parameter dependent dilepton pair production developed in [M. Klusek-Gawenda, WS, A. Szczurek Phys. Lett. B 814 (2021) 136114]. We study the distribution of the angle between difference and sum of lepton transverse momenta, and show how it relates to the orbital angular momentum of leptons. The dependence on impact paramter is discussed, and we also present the different components of the Wigner function in the t-channel. A brief comparison to similar angular distributions in diffractive quark pair production will be presented.
Copyright (c) 2024 Wolfgang Schäfer https://creativecommons.org/licenses/by/4.0
Coherent photoproduction of J/ψ mesons in ultraperipheral Pb–Pb collisions at CMS
In this contribution, we briefly describe the CMS results on J/ψ photoproduction in Pb–Pb collisions, in which the coherent J/ψ photoproduction cross section as a function of the photon-nucleus center-of-mass energy (WγN^Pb) was measured up to 400 GeV. The cross section is observed to plateau above WγN^Pb ≈ 40 GeV and up to 400 GeV, accessing very small Bjorken-x values of ≈ 6 × 10⁻⁵.
Copyright (c) 2024 L. F. Alcerro (for the CMS Collaboration) https://creativecommons.org/licenses/by/4.0
Quantum mechanical aspects of coherent photoproduction: the limits of coherence, and multiple vector mesons
Quantum mechanics is central to coherent photoproduction in ultra-peripheral collisions (UPCs). This writeup will discuss some surprising aspects of UPCs that stem from these quantum mechanical roots. The Good-Walker (GW) paradigm, which connects coherent photoproduction with the target nucleus remaining in its ground state. This contrasts with a semi-classical picture, where coherence depends on the positions of the individual nucleons and the momentum transfer. Unlike the GW approach, the semiclassical picture is consistent with the observed data on coherent photoproduction with nuclear breakup, and with coherent photoproduction in peripheral collisions. The semiclassical approach allows for a wider variety of coherent UPC reactions, such as coherent photoproduction of charged mesons, including some non-quark-antiquark exotica. Quantum mechanics is also key to the coherent photoproduction of multiple vector mesons by the interactions of a single ion pair. The vector mesons share a common impact parameter, and so can exhibit richer interference patterns than single mesons. At forward rapidities, the cross sections to produce multiple identical vector mesons are enhanced due to superradiance. With enough statistics, multi-meson events may provide an opportunity to observe stimulated decays.
Copyright (c) 2024 Spencer R. Klein https://creativecommons.org/licenses/by/4.0
K+K− photoproduction in ultra-peripheral Pb–Pb collisions
In ultra-peripheral collisions (UPCs) of relativistic heavy ions, photoproduction occurs when a photon emitted from one nucleus interacts with the other nucleus from the opposing beam, producing particles in the final state. Measurements of KK photoproduction probe interactions and couplings between the phi (1020) and charged kaons with photons and nuclear targets. We report the exclusive KK photoproduction cross section at midrapidity in Pb–Pb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV, which is measured for the first time in UPCs.
Copyright (c) 2024 Minjung Kim (for the ALICE Collaboration) https://creativecommons.org/licenses/by/4.0
Recent results on ultra-peripheral collisions with the ALICE experiment
Recent measurements of J/ψ photoproduction based on data from ultra-peripheral Pb–Pb and p–Pb collisions recorded by the ALICE experiment during Run 2 of the LHC are presented. Photoproduction, as a photon-induced process, is sensitive to the structure of hadrons, and the results are of great importance for a better understanding of how gluon saturation and nuclear shadowing function in high-energy quantum chromodynamics. The energy evolution of coherent J/ψ photoproduction has been measured, indicating a strong suppression of nuclear gluon densities at high energies. The energy dependence has also been measured for exclusive and dissociative J/ψ production off the proton. The average distribution of nuclear gluon matter in the transverse plane, along with its fluctuations, has been studied for the first time using measurements of the dependence of coherent and incoherent J/ψ photoproduction on the transferred momentum t. Lastly, the data from coherent photoproduction of J/ψ in Pb–Pb ultra-peripheral collisions have been found to be compatible with a transverse polarization.
Copyright (c) 2024 David Grund (for the ALICE Collaboration) https://creativecommons.org/licenses/by/4.0
Vector meson production using the Balitsky-Kovchegov equation including the dipole orientation
In this proceedings a solution of the target-rapidity Balitsky-Kovchegov (BK) equation is presented considering the complete impact-parameter dependence, including the orientation of the dipole with respect to the impact-parameter vector. The target-rapidity formulation of the BK equation introduces non-locality in rapidity. Three different prescriptions are considered to take into account the rapidities preceding the initial condition. The solutions are used to compute the structure functions of the proton and the diffractive photo- and electro-production of $J/\psi$ off protons. The predictions agree well with HERA data, confirming that the target-rapidity Balitsky-Kovchegov equation with the full impact-parameter dependence is a viable tool to study the small Bjorken-$x$ limit of perturbative QCD at current facilities like RHIC and LHC as well as in future colliders like the EIC.
Copyright (c) 2024 J. Cepila, J. G. Contreras, M. Vaculciak https://creativecommons.org/licenses/by/4.0
Vector meson production in ultraperipheral heavy ion collisions
We review model calculations of exclusive vector meson production in ultraperipheral heavy ion collisions. We highlight differences and similarities between different dipole models and leading twist shadowing calculations. Recent color glass condensate calculations are presented with focus on effects from nuclear structure and azimuthal anisotropies driven by interference effects.
Copyright (c) 2024 B. Schenke , H. Mantysaari, F. Salazar, C. Shen, W. Zhao https://creativecommons.org/licenses/by/4.0
Historical introduction to ultra peripheral collisions
This is a brief history of photons, both soft and hard, real and virtual. About 150-100 years ago, Maxwell and Einstein discovered intriguing properties of electromagnetic fields and how to understand them both macroscopically and microscopically. Decades later, physicists developed the theory of renormalized quantum electrodynamics (QED), an incredibly accurate theory describing interactions of photons and other particles. Photons are used everywhere in academia and technological devices, from supermarket lasers and doors to academic studies in atomic, nuclear, and particle physics. In this article, I attempt to convey how the field of relativistic heavy ions rediscovered ultra-peripheral collisions (UPC) as a source of intense, almost real photons, and how it permits the study of a plethora of phenomena in the aforementioned academic fields. These phenomena are not always accessible by other means.
Copyright (c) 2024 Carlos Bertulani https://creativecommons.org/licenses/by/4.0
Measurement of the impact-parameter dependent azimuthal anisotropy in coherent ρ0 photoproduction with ALICE
Coherent vector meson photoproduction in ultraperipheral heavy-ion collisions is a well established tool to probe the gluon structure of the colliding nuclei. We will focus on the observation of quantum interference effects in the ρ 0 meson photoproduction, in the form of angular anisotropy. Such an anisotropy appears due to two different factors: first, the photons involved in the process are linearly polarized along the impact parameter, and, second, quantum interference occurs between the two amplitudes that contribute to the ρ0 photoproduction cross section. Furthermore, the interference effect strongly depends on the impact parameter of the collision, which acts as the distance between the openings of a two-slit interferometer. We present the first measurement of this anisotropy in coherent ρ0 photoproduction from ultraperipheral Pb–Pb collisions at a center-of-mass energy of √sNN = 5.02 TeV per nucleon pair, as a function of the impact parameter of the collision. The latter is estimated by classifying the events in nuclear-breakup classes defined by neutron emission. The ρ0 mesons are detected by the ALICE experiment through their decay into a pion pair. The anisotropy occurs as a function of ϕ, defined as the azimuth angle between the two vectors formed by the sum, and the difference, of the transverse-momentum of the pions, respectively. It results in a cos(2ϕ) modulation of the photoproduced ρ0; the amplitude of the modulation is found to increase by about one order of magnitude from large to small impact parameters. This trend is compatible with the available theoretical predictions
Copyright (c) 2024 Andrea G. Riffero (for the ALICE Collaboration) https://creativecommons.org/licenses/by/4.0