Medium-induced modification of $Z$-tagged charged particle yields in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

The yield of charged particles opposite to a $Z$ boson with large transverse momentum ($p_{T}$) is measured in 260 pb$^{-1}$ of $pp$ and 1.7 nb$^{-1}$ of Pb+Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The $Z$ boson tag is used to select hard-scattered partons with specific kinematics, and to observe how their showers are modified as they propagate through the quark-gluon plasma created in Pb+Pb collisions. Compared with $pp$ collisions, charged-particle yields in Pb+Pb collisions show significant modifications as a function of charged-particle $p_{T}$ in a way that depends on event centrality and $Z$ boson $p_{T}$. The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a $p_{T}$ regime difficult to measure through other channels.