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

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, $R_{\mathrm{C}\mathrm{P}}$, is evaluated, and the results are presented as a function of variables that reflect the kinematics of the initial hard parton scattering process. The $R_{\mathrm{C}\mathrm{P}}$ 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 $x_{\mathrm{P}\mathrm{b}}$. 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 $x_{\mathrm{P}\mathrm{b}}\sim 4\cdot {10}^{-4}$.