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 (pT) is measured in 260 pb−1 of pp and 1.7 nb−1 of Pb þ 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 quarkgluon plasma created in Pb þ Pb collisions. Compared with pp collisions, charged-particle yields in Pb þ Pb collisions show significant modifications as a function of charged-particle pT in a way that depends on event centrality and Z boson pT . The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a pT regime difficult to measure through other channels.