Study of Jet Quenching with $ℤ+jet$ Correlations in Pb-Pb and pp Collisions at $\sqrt{s_{NN}} = 5.02$ TeV

The production of jets in association with Z bosons, reconstructed via the $μ^+μ^−$ and $e^+e^−$ decay channels, is studied in pp and, for the first time, in Pb-Pb collisions. Both data samples were collected by the CMS experiment at the LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The Pb-Pb collisions were analyzed in the 0%–30% centrality range. The back-to-back azimuthal alignment was studied in both pp and Pb-Pb collisions for Z bosons with transverse momentum $p^Z_T>60$ GeV/c and a recoiling jet with $p^{jet}_T>30$ GeV/c. The $p_T$ imbalance $x_{jZ}=p^{jet}_T/p^Z_T$, as well as the average number of jet partners per $Z, R_{jZ}$, was studied in intervals of $p^Z_T$. The $R_{jZ}$ is found to be smaller in Pb-Pb than in pp collisions, which suggests that in Pb-Pb collisions a larger fraction of partons associated with the Z bosons fall below the 30 GeV/c $p^{jet}_T$ threshold because they lose energy