Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions

Abstract

The first measurement of pseudorapidity and azimuthal angle distributions relative to the momentum vector of a Z boson for low transverse momentum (p_T) charged hadrons in lead-lead (PbPb) collisions is presented. By studying the hadrons produced in an event with a high-p_T Z boson (40 < p_T < 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 √|b|𝒔_NN = 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 p_T. A significant modification of the distributions for charged hadrons in the range 1 < p_T < 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. When compared with theoretical predictions, the results are consistent with expectations of a hydrodynamic wake created when the QGP is depleted of energy by the parton propagating through it. Therefore, this Letter presents the first evidence of probe-induced energy depletion and the resulting response by the QGP medium.