Observation of medium induced modifications of jet fragmentation in PbPb collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV using isolated-photon-tagged jets

TL;DR

This study measures how the quark-gluon plasma in PbPb collisions modifies jet fragmentation functions using isolated-photon-tagged jets, revealing medium-induced changes not seen in peripheral collisions, providing a new reference for QGP models.

Contribution

First measurement of jet fragmentation functions in PbPb collisions with isolated-photon tagging, showing medium modifications of parton showers with well-defined initial conditions.

Findings

Central PbPb collisions show altered fragmentation functions compared to pp.

Low pT particles are enhanced, high pT particles are depleted in central PbPb.

No significant modifications are observed in peripheral collisions.

Abstract

Measurements of fragmentation functions for jets associated with an isolated photon are presented for the first time in pp and PbPb collisions. The analysis uses data collected with the CMS detector at the CERN LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are obtained for jets with p$_\mathrm{T}^\text{jet} >$ 30 GeV in events containing an isolated photon with p$_\mathrm{T}^\gamma>$ 60 GeV, using charged tracks with transverse momentum p$_\mathrm{T}^\text{trk} >$ 1 GeV in a cone around the jet axis. The association with an isolated photon constrains the initial p$_\mathrm{T}$ and azimuthal angle of the parton whose shower produced the jet. For central PbPb collisions, modifications of the jet fragmentation functions are observed when compared to those measured in pp collisions, while no significant differences are found in the 50% most peripheral collisions. Jets in central PbPb events show an excess (depletion) of low (high) p$_\mathrm{T}$ particles, with a transition around 3 GeV. This measurement shows for the first time the in-medium shower modifications of partons (quark dominated) with well defined initial kinematics. It constitutes a new well-controlled reference for testing theoretical models of the parton passage through the QGP.