Using Z boson events to study parton-medium interactions in PbPb collisions

TL;DR

This paper investigates how the presence of a quark-gluon plasma in PbPb collisions affects the behavior of parton showers by analyzing charged hadron spectra associated with Z boson events, revealing medium-induced modifications.

Contribution

It provides the first detailed measurements of charged hadron spectra and angular correlations in Z boson events in PbPb collisions, highlighting medium modifications of parton showers.

Findings

Modification of azimuthal angular distributions in PbPb collisions

Alterations in fragmentation functions and $p_T$ spectra in PbPb

Evidence of medium effects on parton shower structure

Abstract

The spectra measurements of charged hadrons produced in the shower of a parton originating in the same hard scattering with a leptonically decaying Z boson, are reported in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Both PbPb and pp data sets are recorded by the CMS experiment at the LHC, and correspond to an integrated luminosity of 1.7 nb$^{-1}$ and 320 pb$^{-1}$, respectively. Hadronic collision data with one reconstructed Z boson candidate with the transverse momentum $p_\mathrm{T}$ $\gt$ 30 GeV/$c$ are analyzed. The Z boson constrains the initial energy and direction of the associated parton. In heavy ion events, azimuthal angular distributions of charged hadrons with respect to the direction of a Z boson are sensitive to modifications of the in-medium parton shower and medium response. Compared to reference data from pp interactions, the results for central PbPb collisions indicate a modification of the angular correlations. The measurements of the fragmentation functions and $p_\mathrm{T}$ spectra of charged particles in Z boson events, which are sensitive to medium modifications of the parton shower longitudinal structure, are also reported. Significant modifications in central PbPb events compared to pp reference data are also found for these observables.