Abstract
The exclusive photoproduction of the heavy vector mesons $\psi$'s and $Y$'s is investigated in peripheral lead-lead collisions for the energies available at the LHC, $\sqrt{s}=2.76$ TeV and $\sqrt{s}=5.02$ TeV. In order to evaluate the robustness of the light-cone color dipole formalism, previously tested in the ultraperipheral regime. It was calculated the rapidity distribution as well as the nuclear modification factor ($R_{AA}$) for the three centrality classes: 30%-50%, 50%-70% and 70%-90%. The transition from ultraperipheral to peripheral regime was carried out sophisticating the photon flux description and the photonuclear cross section, taking into account the effective interaction area. In our calculations, three scenarios were considered: (scenario 1) the direct application of the usual photon flux and of the photonuclear cross section without any relevant change in relation to the UPC's, (scenario 2) the application of an effective photon flux keeping the photonuclear cross section unchanged and (scenario 3) where it was also considered an effective photonuclear cross section. The results obtained from the three scenarios were compared with the ALICE measurements (only $J/\psi$ at the moment), showing a better agreement with the data in the more complete approach (scenario 3), mainly in the more central regions (30%-50% and 50%-70%) where the incertainty is smaller.
Highlights
One of the main contributions of HERA was the discovery of diffractive events, characterized by large rapidity gaps (η ≳ 4) with absence of hadronic activity, where one or both hadrons emerge intact in the final state, representing a relevant fraction of deep inelastic scattering [1]
The main goal of this work is to evaluate the behavior of the nuclear modification factor in three different approaches: direct application of the usual photon flux and of the photonuclear cross section without any modification in relation to UPCs, altering only in the photon flux, and modifying the photon flux and in the photonuclear cross section
Integrating in the impact parameter for the centrality classes (30%–50%, 50%–70%, and 70%–90%), the results shown in Table I, which presents the ALICE data, are obtained
Summary
One of the main contributions of HERA was the discovery of diffractive events, characterized by large rapidity gaps (η ≳ 4) with absence of hadronic activity, where one or both hadrons emerge intact in the final state, representing a relevant fraction of deep inelastic scattering [1]. Soft diffraction events contribute with (∼20%) of the total inelastic proton-proton cross section and must be taken into account in order to keep the background of many processes in the LHC [2,3] under control. The hard diffractive processes, responsible for the production of the states with high mass, or high pT (for example, QQ , jets, W, and Z), can be calculated from perturbative QCD.
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