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. Exclusive photoproduction of J/ψ vector mesons is sensitive to the gluon distribution of the target. Here we report on the ALICE measurement of J/ψ coherent photoproduction in Pb-Pb ultra-peripheral collisions at √ sNN = 2.76 TeV for the rapidity ranges -3.6 <y< -2.6 and |y| < 0.9, and on preliminary results on the J/ψ photoproduction in p-Pb at √ sNN = 5.02 TeV. The J/ψ mesons have been identified through their leptonic decays.
Highlights
The photonuclear reactions involving heavy vector meson exclusive production provide a good tool to test the behavior of the gluon distribution functions at low Bjorken-x, because the photoproduction cross section scales at leading order as the square of the gluon PDF
As the hadronic interactions are suppressed, the only mechanism in question is via the strong electromagnetic field, whose intensity is proportional to the square of the electric charge of the ion
The ALICE experiment consists of central tracking detectors covering pseudorapidity |η| < 0.9, forward muon spectrometer at rapidity range -4.0 < η < -2.5, forward scintillator arrays on both sides at pseudorapidities 2.8 < η < 5.1 and -3.7 < η < -1.7 and zero degree neutron and proton calorimeters placed on both sides at a distance of ±116 meters from the nominal ALICE interaction point
Summary
The photonuclear reactions involving heavy vector meson exclusive production provide a good tool to test the behavior of the gluon distribution functions at low Bjorken-x, because the photoproduction cross section scales at leading order as the square of the gluon PDF. At the LHC it is possible to study photonuclear reactions at higher energies than ever before, via ultra-peripheral collisions (UPC) In these interactions the nuclei or protons are passing each other at a distance greater than the sum of the nuclear radii in the ion-ion case, or at impact parameter large enough to strongly suppress the hadronic interactions in the case of ion-proton system or protonproton collisions. Exclusive J/ψ photoproduction has been previously measured in Au-Au collisions at RHIC [4], exclusive J/ψ photoproduction in γp interactions has been studied at HERA [5] [6], in ppcollisions at Tevatron [7] and recently LHCb reported exclusive J/ψ and ψ(2S ) production in pp collisions [8]
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