Abstract
Ultra-relativistic heavy ions generate strong electromagnetic fields which offer the possibility to study gamma-gamma, gamma-nucleus and gamma-proton processes at the LHC in ultra-peripheral Pb-Pb and p-Pb collisions (UPC). Exclusive photoproduction of J/ψ vector mesons is sensitive to the gluon distribution of the interacting target (proton or nucleus). This process is expected to be sensitive to saturation phenomena and nuclear gluon shadowing. Here we report on the ALICE measurement of J/ψ coherent photoproduction in Pb-Pb UPC at sNN=2.76 TeV at forward and central rapidities. Furthermore, we also present results on the J/ψ photoproduction in p-Pb UPC at sNN=5.02 TeV in the forward and backward rapidities, where the rapidity is measured in the laboratory frame with respect to the proton beam direction. The J/ψ mesons have been identified through their leptonic decays.
Highlights
Photoproduction reactions of photons on protons or lead nuclei, γp → J/ψ p and γPb → J/ψ Pb, which leave only a J/ψ meson as a new particle in the final state, allow the study of saturation phenomena and nuclear gluon shadowing at low Bjorken-x
The cross section of exclusive photoproduction of J/ψ in LO pQCD is sensitive to the square of the gluon distribution in the target [1]
The ALICE experiment measured this process in γp and γPb with ultra-peripheral p-Pb and Pb-Pb collisions (UPC)
Summary
Photoproduction reactions of photons on protons or lead nuclei, γp → J/ψ p and γPb → J/ψ Pb, which leave only a J/ψ meson as a new particle in the final state, allow the study of saturation phenomena and nuclear gluon shadowing at low Bjorken-x. Is through the electromagnetic field, which behaves as a flux of virtual photons The intensity of such a flux is proportional to the square of the electric charge. Pb-Pb UPC allow us to study γ-Pb reactions, since both of the nuclei can be the source of virtual photons. Exclusive J/ψ production has been measured at HERA [3] [4] using ep collisions, at the Tevatron [5] in ppcollisions at mid-rapidity and recently by LHCb in pp collisions at forward rapidity [6]. Adam / Nuclear and Particle Physics Proceedings 273–275 (2016) 2599–2601 production at forward rapidities [7] and coherent and incoherent J/ψ production at mid-rapidities [8] in Pb-Pb
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