J/Ψ production in ultra-peripheral proton-lead and lead-lead collisions with CMS

Abstract

This thesis reports on photoproduction of exclusive J/ψ mesons in ultra-peripheral PbPb and pPb collisions at √sNN = 2.76 TeV, and 5.02 TeV, respectively, from the CMS experiment at the LHC. For PbPb collisions, there is a correlation between the directions of the produced J/ψ and the neutron emitted by the struck nucleus. For J/ψ mesons with pT 100 MeV/c, the ratio of the pT distributions for J/ψ is moving in the opposite direction as the neutrons emitted from the target nucleus to those for J/ψ is moving in the same direction as the neutrons, suggests significant shadowing in the lead nucleus. For pPb collisions, the distributions of the J/ψs in transverse momentum have a similar form to previous measurements at HERA. The average pT increases with the photon-proton center of mass energy W . This implies that the spatial distribution of the gluons within the proton depends upon the gluon momentum. The cross-section also increases as W increases from 57 to 439 GeV implying that the proton becomes more and more opaque to photons as the photon energy increases. Parametrizing the growth ofthecrosssectionbytheformσ(W)∝Wδ yieldsavalueofδ =0.636±0.063. This result is consistent with the world average and has a precision comparable to other experiments.