Measurement of the centrality dependence of the dijet yield in $p$+Pb collisions at $\sqrt{s_{_\text{NN}}}$ = 8.16 TeV with the ATLAS detector

TL;DR

This paper measures how the yield of dijet events in proton-lead collisions varies with collision centrality, revealing dependence on proton-side parton momentum fractions and providing insights into small proton configurations.

Contribution

It presents the first detailed measurement of the centrality dependence of dijet yields in p+Pb collisions at 8.16 TeV, focusing on parton momentum fractions from valence to small-x regions.

Findings

$R_{CP}$ scales with proton-side Bjorken-x, $x_p$.

No $R_{CP}$ trend observed with lead-side $x_{Pb}$.

Provides data on parton momentum fractions down to $10^{-3}$.

Abstract

The measurement of hard scatterings in proton-nucleus collisions has resulted in a greater understanding of both the proton and nuclear structure. ATLAS measured the centrality dependence of the dijet yield using 165 nb$^{-1}$ of $p$+Pb data collected at $\sqrt{s_{_\text{NN}}}$ = 8.16 TeV in 2016. The event centrality, which reflects the $p$+Pb impact parameter, is characterized by the total transverse energy registered in the Pb-going side of the forward calorimeter. The central-to-peripheral ratio of the scaled dijet yields, $R_\mathrm{CP}$, is evaluated, and the results are presented as a function of variables that reflect the kinematics of the initial hard parton scattering process. The $R_\mathrm{CP}$ shows a scaling with the Bjorken-$x$ of the parton originating from the proton, $x_p$, while no such trend is observed as a function of $x_\mathrm{Pb}$. This analysis provides unique input to understanding the role of small proton spatial configurations in $p$+Pb collisions by covering parton momentum fractions from the valence region down to $x_p \sim 10^{-3}$ and $x_\mathrm{Pb}\sim 4\cdot10^{-4}$.