Constraining Gluon Distributions in Nuclei Using Dijets in Proton-Proton and Proton-Lead Collisions at √sNN=5.02 TeV

Abstract

The pseudorapidity distributions of dijets as functions of their average transverse momentum ( p ave T ) are measured in proton-lead ( p Pb ) and proton-proton ( p p ) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. A significant modification of the p Pb spectra with respect to the p p spectra is observed in all p ave T intervals investigated. The ratios of the p Pb and p p distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken x in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.