Speaker
Description
This talk presents the first measurement of low-transverse-momentum ($p_T$) charged-hadron distributions in pseudorapidity and azimuthal angle, relative to the momentum direction of $Z$ bosons, in lead-lead ($\mathrm{PbPb}$) collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}} = 5.02$ TeV. The analysis uses PbPb data from 2018 with an integrated luminosity of $1.67 \pm 0.03$ nb$^{-1}$, complemented by proton-proton (pp) data from 2017 with $301 \pm 6$ pb$^{-1}$. Events are selected with at least one $Z$ boson having $40 < p_\mathrm{T} < 350$ GeV, and charged-hadron distributions are examined in $p_\mathrm{T}$ bins to investigate potential in-medium parton-shower modifications and medium-recoil effects.
A significant modification is observed in the azimuthal and pseudorapidity distributions of charged hadrons with $p_\mathrm{T}$ in the range of 1–2 GeV in $\mathrm{PbPb}$ collisions compared to pp references. These results align with phenomenological models, such as the Co-LBT Hydro and HYBRID models, that incorporate the positive and negative wake effects associated with the fast moving-quarks and gluons in the Quark-Gluon Plasma (QGP).
This analysis provides crucial new insights into the interplay between hard and soft particle production in heavy-ion collisions. By comparing PbPb data with pp baselines and theoretical predictions, the findings offer the first experimental evidence for medium-recoil and medium-hole effects induced by a hard probe, contributing to a deeper understanding of jet quenching mechanisms in the QGP.
