CMS-PAS-HIN-24-014

CMS-PAS-HIN-24-014
Constraining nPDFs using dijet production in pPb collisions at 8.16 TeV with the CMS experiment
CMS Collaboration
24 November 2024

Abstract: The dijet pseudorapidity ($ \eta^{\text{dijet}} $) distribution in proton-lead (pPb) collisions is sensitive to nuclear modifications of parton distribution functions (PDFs). Additionally, the measurement of the $ \eta^{\text{dijet}} $ dependence of the average transverse momenta of dijets provides a precise method for investigating the energy scale of partonic interactions and their momentum transfer dependence ($ Q^{2} $). Previously published measurements of dijet production in pPb collisions at a center-of-mass collision energy per nucleon pair of $ \sqrt{s_{\mathrm{NN}}}= $ 5.02 TeV were used to constrain nuclear PDF fits based on next-to-leading order calculations. Here, a new measurement of dijet distributions using pPb collision data collected at a higher energy of 8.16 TeV is reported. The data, recorded by the CMS experiment at the LHC, correspond to an integrated luminosity of 174.6 nb$^{-1} $. The $ \eta^{\text{dijet}} $ distributions are measured for multiple dijet average transverse momentum $ p_{\text{T}}^{\text{ave}} $ intervals from 50 to 500 GeV.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Measured $ \eta^{\text{dijet}} $ distributions in different bins of the dijet average transverse momentum, $ p_{\mathrm{T}}^{\text{ave}} $. The shaded areas represent the systematic uncertainties, and the error bars show the statistical uncertainties.
png pdf	Figure 2: Measured $ \eta^{\text{dijet}} $ distributions in different bins of the dijet average transverse momentum, $ p_{\mathrm{T}}^{\text{ave}} $. The shaded areas represent the systematic uncertainties, and the error bars show the statistical uncertainties.
png pdf	Figure 3: Measured forward-to-backward $ \eta^{\text{dijet}}_{\mathrm{CM}} $ ratios in different bins of $ p_{\mathrm{T}}^{\text{ave}} $. The shaded areas represent the systematic uncertainties, and the error bars show the statistical uncertainties.
png pdf	Figure 4: Measured forward-to-backward $ \eta^{\text{dijet}}_{\mathrm{CM}} $ ratios in different bins of $ p_{\mathrm{T}}^{\text{ave}} $. The shaded areas represent the systematic uncertainties, and the error bars show the statistical uncertainties.

Summary

In summary, we report measurements of the dijet pseudorapidity, $ \eta^{\text{dijet}} $, across various average transverse momentum intervals, $ p_{\mathrm{T}}^{\text{ave}} $, intervals in $ \mathrm{p}\text{Pb} $ collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 8.16 TeV. The measured distributions can be compared to theoretical calculations performed with nuclear parton distribution functions (nPDFs). This dataset explores lower Bjorken $ x $ values compared to prior dijet analyses, thereby accessing a previously unconstrained region of the gluon nPDF via dijet final states. It establishes new constraints on existing nPDF models, crucial for advancing our understanding of high-$ p_{\mathrm{T}} $ and high-mass particle production at the LHC energies.

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