Search

In ultraperipheral heavy-ion collisions (UPCs) at the Large Hadron Collider (LHC) Pb nuclei are excited through interactions induced by strong electromagnetic fields. The expected excitation energy could reach hundreds MeV, which leads to the subsequent emission of various particles, including neutrons, protons, and alpha particles. To accurately describe deexcitation of nuclei, we have developed two novel approaches. The first method utilizes the Heavy Ion Phase Space Exploration (HIPSE) model to simulate pre-equilibrium emissions and to estimate the excitation energy of the remaining nucleus. Our second approach introduces a new technique for modeling the excitation energy of the nucleus. This method consists of a two-component function to represent the excitation energy distribution more precisely, accounting for the energy loss due to the interaction between photons and quasi-deuteron. Both of these modeling techniques are integrated with the results produced by the GEMINI++ generator, which implements the Hauser-Feshbach formalism to simulate the statistical decay of excited nuclei. The alternative calculations are done with EMPIRE platform. Using these approaches, we obtained the cross sections for the emission of neutrons, protons, and alpha particles resulting from UPC at the LHC. Our results were compared with the experimental data of the ALICE group on neutron and proton multiplicities., Comment: 6 pages, Presented at the 57th Zakopane Conference on Nuclear Physics, "Extremes of the Nuclear Landscape", Zakopane, Poland, 25 August-1 September, 2024

We discuss possible future studies of photon-photon (light-by-light) scattering using a planned FoCal and ALICE 3 detectors. We include different mechanisms of $\gamma\gamma\to\gamma\gamma$ scattering, such as double-hadronic photon fluctuations, $t/u$-channel neutral pion exchange or resonance excitations ($\gamma \gamma \to R$) and deexcitation ($R \to \gamma \gamma$). The broad range of (pseudo)rapidities and lower cuts on transverse momenta open a necessity to consider not only dominant box contributions but also other subleading contributions. Here we include low mass resonant $R = \pi^0$, $\eta$, $\eta'$ contributions. The resonance contributions give intermediate photon transverse momenta. However, these contributions can be eliminated by imposing windows on di-photon invariant mass. We study and quantify individual box contributions (leptonic, quarkish). The electron/positron boxes dominate at low $M_{\gamma \gamma}<1$ GeV di-photon invariant masses. The PbPb$\to$PbPb$\gamma \gamma$ cross section is calculated within equivalent photon approximation in the impact parameter space. Several differential distributions are presented and discussed. We consider four different kinematic regions. We predict 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-$\pi^0$ background which can, in principle, be eliminated at the new kinematical range for the ALICE 3 measurements by imposing dedicated cuts on di-photon transverse momentum and\or so-called vector asymmetry., Comment: 20 pages, 19 Figures, 3 Tables; misprint corrected, 2 figures added

This note represents an attempt to gather the input related to light-by-light scattering ($\gamma\gamma$) cross-section measurements at LHC with the aim of checking the consistency with different standard model predictions. For the first time, we also consider the contribution from the $\eta_b(1S)$ meson production to the diphoton invariant mass distribution, by calculating its inclusive photoproduction cross-section. Using a simplified set of assumptions, we find a result of $115\pm 19\,\,\text{nb}$, consistent with standard model predictions within two standard deviations. Although an improved determination of the integrated fiducial $\textrm{PbPb}\,(\gamma\gamma)\to \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,\gamma\gamma$ cross-section by approximately 10\% could be potentially achieved relative to current measurements, further improvements are expected with the inclusion of existing or forthcoming LHC nuclear data.

The future opportunities for high-density QCD studies with ion and proton beams at the LHC are presented. Four major scientific goals are identified: the characterisation of the macroscopic long wavelength Quark-Gluon Plasma (QGP) properties with unprecedented precision, the investigation of the microscopic parton dynamics underlying QGP properties, the development of a unified picture of particle production and QCD dynamics from small (pp) to large (nucleus--nucleus) systems, the exploration of parton densities in nuclei in a broad ($x$, $Q^2$) kinematic range and the search for the possible onset of parton saturation. In order to address these scientific goals, high-luminosity Pb-Pb and p-Pb programmes are considered as priorities for Runs 3 and 4, complemented by high-multiplicity studies in pp collisions and a short run with oxygen ions. High-luminosity runs with intermediate-mass nuclei, for example Ar or Kr, are considered as an appealing case for extending the heavy-ion programme at the LHC beyond Run 4. The potential of the High-Energy LHC to probe QCD matter with newly-available observables, at twice larger center-of-mass energies than the LHC, is investigated., Comment: 209 pages; Report from Working Group 5 of the Workshop on the Physics of the CERN HL-LHC, and Perspectives at the HE-LHC (v2: Minor updates to text and figures. Added one author.)

We discuss nuclear cross sections for $AA \to AAV$ and $AA \to AAVV$ reactions with one or two vector mesons in the final state. Our analysis is done in the impact parameter space equivalent photon approximation. This approach allows to consider the above processes taking into account distance between colliding nuclei. We consider both ultraperipheral and semi-central collisions. We are a first group which undertook a study of single $J/\psi$ photoproduction for different centrality bins. We show that one can describe new ALICE experimental data by including geometrical effects of collisions in the flux factor. Next, total and differential cross section for double-scattering mechanism in the exclusive $AA \to AAVV$ reaction in ultrarelativistic ultraperipheral heavy ion collisions is presented. In this context we consider double photoproduction and photon-photon processes. Simultaneously, we get very good agreement of our results with STAR (RHIC), CMS and ALICE (LHC) experimental data for single $\rho^0$ and $J/\psi$ vector meson production. A comparison of our predictions for exclusive four charged pions production is also presented., Comment: 6 pages, 6 figures, an invited talk presented by M. Klusek-Gawenda at the 14th International Workshop on Meson Production, Properties and Interaction, (Meson2016), Krakow, Poland, June 2-7, 2016

Multiple Partonic Interactions are often crucial for interpreting results obtained at the Large Hadron Collider (LHC). The quest for a sound understanding of the dynamics behind MPI - particularly at this time when the LHC is due to start its "Run II" operations - has focused the aim of this workshop. MPI@LHC2014 concentrated mainly on the phenomenology of LHC measurements whilst keeping in perspective those results obtained at previous hadron colliders. The workshop has also debated some of the state-of-the-art theoretical considerations and the modeling of MPI in Monte Carlo event generators. The topics debated in the workshop included: Phenomenology of MPI processes and multiparton distributions; Considerations for the description of MPI in Quantum Chromodynamics (QCD); Measuring multiple partonic interactions; Experimental results on inelastic hadronic collisions: underlying event, minimum bias, forward energy flow; Monte Carlo generator development and tuning; Connections with low-x phenomena, diffraction, heavy-ion physics and cosmic rays. In a total of 57 plenary talks the workshop covered a wide range of experimental results, Monte Carlo development and tuning, phenomenology and dedicated measurements of MPI which were produced with data from the LHC's Run I. Recent progress of theoretical understanding of MPI in pp, pA and AA collisions as well as the role of MPI in diffraction and small-x physics were also covered. The workshop forstered close contact between the experimental and theoretical communities. It provided a setting to discuss many of the different aspects of MPI, eventually identifying them as a unifying concept between apparently different lines of research and evaluating their impact on the LHC physics programme., Comment: MPI@LHC 2014, 3 - 7 November 2014, Krakow, Poland, 81 pages

The large and rapidly varying electric and magnetic fields induced by the spectator systems moving at ultrarelativistic velocities induce a charge splitting of directed flow, $v_1$, of positive and negative pions in the final state of the heavy ion collision. The same effect results in a very sizeable distortion of charged pion spectra as well as ratios of charged pions ($\pi^+/\pi^-$) emitted at high values of rapidity. Both phenomena are sensitive to the actual distance between the pion emission site and the spectator system. This distance $d_E$ appears to decrease with increasing rapidity of the pion, and comes below $\sim$1~fm for pions emitted close to beam rapidity. In this paper we discuss how these findings can shed new light on the space-time evolution of pion production as a function of rapidity, and on the longitudinal evolution of the system created in heavy ion collisions., Comment: 8 pages, 3 figures a talk by A. Rybicki at the X Workshop on Particle Correlations and Femtoscopy (WPCF 2014), Gy\"ongy\"os, Hungary, 2014