The first measurement of pseudorapidity and azimuthal angle distributions relative to the momentum vector of a Z boson for low transverse momentum (pT) charged hadrons in lead-lead (PbPb) collisions is presented. By studying the hadrons produced in an event with a high-pT Z boson (40 pT 350 GeV), the analysis probes how the quark-gluon plasma (QGP) medium created in these collisions affects the parton recoiling opposite to the Z boson. Utilizing PbPb data at a nucleon-nucleon center-of-mass energy s₍₍ = 5. 02 TeV from 2018 with an integrated luminosity of 1. 67 nb^-1 and proton-proton (pp) data at the same energy from 2017 with 301 pb^-1, the distributions are examined in bins of charged-hadron pT. A significant modification of the distributions for charged hadrons in the range 1 pT 2 GeV in PbPb collisions is observed when compared to reference measurements from pp collisions. The data provide new information about the correlation between hard and soft particles in heavy ion collisions, which can be used to test predictions of various jet quenching models. The results are consistent with expectations of a hydrodynamic wake created when the QGP is depleted of energy by the parton propagating through it. Based on comparisons of PbPb data with pp references and predictions from theoretical models, this Letter presents the first evidence of medium-recoil and medium-hole effects caused by a hard probe.
CMS Collaboration (2025) studied this question.