# Jet overlap in heavy ion collisions at LHC energies and its consequences on jet shape

## TL;DR

This paper investigates how overlapping jets in central Pb-Pb collisions at LHC energies affect jet identification and shape, revealing significant intruder contamination and its impact on jet measurements.

## Contribution

It provides the first detailed analysis of jet overlap effects in heavy ion collisions at LHC energies using EPOS3 simulations.

## Key findings

- 30% of high-energy jets contain intruder particles from overlaps
- Overlap effects increase with higher beam energy and larger jet cone size
- Intruder particles significantly alter the radial momentum distribution of jets

## Abstract

Central lead-lead collisions at the LHC energies may pose a particular challenge for jet identification as multiple jets are produced per each collision event. We simulate the jet evolution in central Pb-Pb events at $\sqrt{s_{\rm NN}} = 2.76$ GeV collision energy with EPOS3 initial state, which typically contains multiple hard scatterings in each event. Therefore the partons from different jets have a significant chance to overlap in momentum space. We find that 30% of the jets with $p_\perp > 50$ GeV, identified by the standard anti-$k_\perp$ jet finding algorithm with jet cone size R=0.3, contain `intruder' particles from overlapping generator-level jets. This fraction increases with increasing beam energy and increasing R. The reconstructed momentum of the jet differs from that of the modelled jet by the loss due to jet partons which are outside of the jet cone and by the gain due to intruder partons. The sum of both may be positive or negative. These intruder partons particularly affect the radial jet momentum distribution because they contribute mostly at large angles $\Delta r$ with respect to the jet centre. The study stresses the importance of the jet overlap effect emerging in central lead-lead collisions at the LHC energies, while being negligible in peripheral PbPb or $p$Pb/$pp$ collisions.