Measurement of the dijet momentum balance in Pb+Pb and $pp$ collisions at $\sqrt{s_{NN}}$= 5.02 TeV with the ATLAS detector

This note presents measurements of the dijet momentum balance of anti-$k_{t}$ R = 0.4 jets at center-of-mass energy $\sqrt{s_{NN}}$ = 5.02 TeV in Pb+Pb and $pp$ collisions by the ATLAS experiment. The measurement was performed using 1.72 $nb^{-1}$ of Pb+Pb data and 260 $pb^{-1}$ of $pp$ data collected in 2018 and 2017 respectively. This note presents distributions of the unfolded $\frac{1}{\mathrm{N_{pair}}} \frac{\mathrm{dN_{pair}}}{\mathrm{d}x_{J}}$, where $x_J$ is the ratio of the transverse momentum ($p_{\mathrm{T}}$) of the subleading jet to the leading jet, as a function of the $p_{\mathrm{T}}$ of the leading jet and of collision centrality. In $pp$ collisions the $x_J$ distributions reach a maximum at $x_J \approx 1$ and decrease approximately linearly with decreasing $x_J$. In central Pb+Pb collisions for leading jets with $158 < p_{\mathrm{T}} < 200$ GeV, the $x_J$ distributions are approximately constant for $x_J > $ 0.5. Even at the highest leading jet $p_{\mathrm{T}}$ measured, $398 < p_{\mathrm{T}} < 562$ GeV, significant modification from the shape in $pp$ collisions is observed. These measurements are compared with previous measurements at $\sqrt{s_{NN}}$ = 2.76 TeV and are found to agree within the systematic uncertainties. These results will help in understanding the strength and mechanism for jet modifications in the quark-gluon plasma.