Measurement of $Z$-tagged charged-particle yields in 5.02 TeV Pb+Pb and $pp$ collisions with the ATLAS detector

The yields of charged particles azimuthally balanced by a high-transverse-momentum ($p_\mathrm{T}$) $Z$ boson are measured in $pp$ and Pb+Pb collision data recorded by the ATLAS detector at the Large Hadron Collider. The measurement is performed using $260$ pb$^{-1}$ of $pp$ collisions and up to $1.7$ nb$^{-1}$ of Pb+Pb collisions, recorded at a center of mass energy of $5.02$~TeV per nucleon pair in 2017 and 2018, respectively. The charged-particle yield per $Z$ boson is measured as a function of charged-hadron $p_\mathrm{T}$ and the hadron-to-boson $p_\mathrm{T}$ ratio $x_\mathrm{hZ}$. The per-$Z$ yields and their ratio between Pb+Pb and $pp$ collisions, $I_\mathrm{AA}$, are reported in different $Z$ boson $p_\mathrm{T}$ ranges and Pb+Pb centrality selections. The $I_\mathrm{AA}$ is observed to be significantly suppressed at large hadron $p_\mathrm{T}$ or $x_\mathrm{hZ}$, presumably due to interaction of the hard-scattered partons with a hot and dense quark-gluon plasma. The $I_\mathrm{AA}$ results are compared to the predictions of jet modification in several Monte Carlo generators and theoretical calculations.