Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 TeV

Abstract

The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of √ sNN = 5.02 TeV. Cross sections are presented in fve diferent J/ψ rapidity ranges within |y| < 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neu trons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for diferent confgurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 < Wγ Pb,n < 920 GeV, where Wγ Pb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10−5 < x < 3.3 × 10−2 . In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.