# Measurement of jet fragmentation in Pb+Pb and $pp$ collisions at $\sqrt{s_\mathrm{NN}} = 2.76$ TeV with the ATLAS detector

## TL;DR

This study measures how jet fragmentation functions differ between Pb+Pb and pp collisions at 2.76 TeV, revealing medium-induced modifications in heavy-ion collisions using ATLAS data.

## Contribution

It provides the first detailed comparison of jet fragmentation functions in Pb+Pb and pp collisions at this energy, highlighting medium effects on jet structure.

## Key findings

- Modest but significant centrality-dependent modifications in Pb+Pb.
- No strong dependence on jet pT or rapidity, except at high transverse momenta.
- Reduction of high-momentum fragments in forward jets.

## Abstract

The distributions of transverse momentum and longitudinal momentum fraction of charged particles in jets are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet transverse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb$^{-1}$ of Pb+Pb data and 4.0 pb$^{-1}$ of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV per colliding nucleon pair. The distributions measured in pp collisions are used as a reference for those measured in Pb+Pb collisions in order to evaluate the impact on the internal structure of jets from the jet energy loss of fast partons propagating through the hot, dense medium created in heavy-ion collisions. Modest but significant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. No significant dependence of modifications on jet $p_{\mathrm{T}}$ and rapidity selections is observed except for the fragments with the highest transverse momenta for which some reduction of yields is observed for more forward jets.