Photonuclear reactions are induced by the strong electromagnetic field generated by ultrarelativistic heavy-ion collisions. These processes have been extensively studied in ultraperipheral collisions, where the impact parameter is larger than twice the nuclear radius. In recent years, the observation of coherent $J/ψ$ photoproduction at very low transverse momentum has been claimed in nucleus$-$nucleus (A$-$A) collisions with nuclear overlap, based on the measurement of an excess in the $J/ψ$ yield with respect to hadroproduction expectations. Such quarkonium measurements can help to constrain the nuclear gluon distribution at low Bjorken-$x$ and high energy. The photoproduced quarkonium is expected to preserve the polarization of the incoming photon due to the $s$-channel helicity conservation. In this contribution, we report on the new preliminary measurements of the $y$-differential cross section and the first polarization analysis of coherently photoproduced $J/ψ$ in peripheral Pb$-$Pb collisions with the ALICE detector at forward rapidity in the dimuon decay channel. Comparison with models will be shown when available.
Ultra-peripheral collisions (UPCs), studied using the ALICE detector, allow us to investigate the low-$x$ behavior of the gluon distribution of the colliding particles. Two new measurements of coherent J/$ψ$ photoproduction cross section from Pb-Pb UPCs at $\sqrt{s_{\rm NN}}=5.02$ TeV are presented: the first measurement of the $|t|$-dependence of the cross section providing a new tool to investigate the transverse gluonic structure at low Bjorken-$x$ and a rapidity-differential measurement at midrapidity allowing us to provide stringent constraints on nuclear gluon shadowing and saturation mo…
The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of $J/ψ$ vector mesons off proton targets in ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. The e$^+$e$^-$ and $μ^+μ^-$ decay channels are used to measure the cross section as a function of the rapidity of the $J/ψ$ in the range $-2.5 < y < 2.7$, corresponding to an energy in the $γ$p centre-of-mass in the interval $40 < W_{γ\mathrm{p}}<550$ GeV. The measurements, which are consistent with a power law dependence of the exclusive $J/ψ$ photoproduc…
Cross sections for the coherent photoproduction of $ρ^{0}$ vector mesons in ultra-peripheral Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}}= 5.02$ TeV are reported. The measurements, which rely on the $π^+π^-$ decay channel, are presented in three regions of rapidity covering the range $|y|<0.8$. For each rapidity interval, cross sections are shown for different nuclear-breakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coher…
The coherent photoproduction of $\rm{J/ψ}$ and $\rm{ψ'}$ mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $\rm{J/ψ}$ is reconstructed using the dilepton ($l^{+} l^{-}$) and proton-antiproton decay channels, while for the $\rm{ψ'}$, the dilepton and the $l^{+} l^{-} π^{+} π^{-}$ decay channels are studied. The analysis is based on an event sample corresponding to an integrated…
Photoproduction of $J/Ψ$ in peripheral Oxygen-Oxygen collisions
Universidade do Estado de Santa Catarina（圣卡塔琳娜州立大学）；Federal University of Grande Dourados, Faculty of Exact Sciences and Technology（大杜拉多斯联邦大学精确科学与技术学院）；Institute of Physics and Mathematics, Federal University of Pelotas (UFPel)（佩洛塔斯联邦大学物理与数学研究所）
Exclusive dimuon production and coherent charmonium photoproduction at forward rapidity in ultra-peripheral Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV
European Organization for Nuclear Research（欧洲核子研究中心）
The first measurement of the coherent photoproduction of $ρ^{0}$ vector mesons in ultra-peripheral Xe-Xe collisions at $\sqrt{s_{\mathrm{\rm NN}}}= 5.44$ TeV is presented. This result, together with previous HERA $γ$p data and $γ$-Pb measurements from ALICE, describes the atomic number ($A$) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the ${\rm Xe}+{\rm Xe} \to ρ^{0} + {\rm Xe}+{\rm Xe}$ process, measured at midrapidity through the decay channel $ρ^{0}\toπ^…
The $ρ^0$ is copiously photoproduced in ultra-peripheral heavy-ion collisions at the LHC. In this talk, I will present recent results on $ρ$ photoproduction with ALICE, including cross-section measurements from PbPb and XeXe collisions, including a discussion of the production of high-mass final states that decay to $π^+π^-$, and of neutron production that accompanies $ρ$ photoproduction. I will conclude by presenting some prospects for ALICE in Runs 3 and 4.
Probing the neutron-skin thickness through $J/ψ$ photoproduction in ultra-peripheral collisions
Shanghai Institute of Applied Physics, Chinese Academy of Sciences（中国科学院上海应用物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Fudan University（复旦大学）；East China Normal University（华东师范大学）
Gluon density and its distributions inside nuclei and the parton modification of bounded nucleons inside a nucleus, are some of the main standing problems in nuclear and particle physics. In recent years, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing the gluon density, which is based on coherent diffractive vector-meson productions, e.g., $J/ψ$ meson. For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC $J/ψ$ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis, we aim to lo…
Measurement of jet photoproduction in ultra-peripheral Pb+Pb collisions without nuclear breakup at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with the ATLAS detector
EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)（欧洲核子研究中心）
