Constraining gluon distributions in nuclei using dijets in proton-proton and proton-lead collisions at $\sqrt{s_{_\mathrm{NN}}} =$ 5.02 TeV

TL;DR

This study measures dijet distributions in proton-proton and proton-lead collisions at 5.02 TeV to investigate modifications in gluon distributions within nuclei, providing evidence of strong suppression at large Bjorken x in lead ions.

Contribution

It presents the first experimental evidence of significant suppression of the gluon PDF at large x in lead nuclei using dijet measurements at the LHC.

Findings

pPb spectra are significantly modified compared to pp spectra.

Gluon PDF at large x in lead is strongly suppressed.

Results agree with nuclear PDF models predicting suppression.

Abstract

The pseudorapidity distributions of dijets as a function of their average transverse momentum ($p_\mathrm{T}^\text{ave}$) are measured in proton-lead (pPb) and proton-proton (pp) 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 pPb spectra with respect to the pp spectra is observed in all $p_\mathrm{T}^\text{ave}$ intervals investigated. The ratios of the pPb and pp 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.