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

## Abstract

The pseudorapidity distributions of dijets as functions of their average transverse momentum $(_{\mathrm{T}}^{ave})$ 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 $P_{\mathrm{T}}^{ave}$ 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.

## Abstract

The pseudorapidity distributions of dijets as functions of their average transverse momentum $(_{\mathrm{T}}^{ave})$ 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 $P_{\mathrm{T}}^{ave}$ 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.

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