Jet shapes in pp and Pb-Pb collisions at ALICE

TL;DR

This paper investigates how the quark-gluon plasma medium modifies the internal structure of jets in Pb-Pb collisions compared to proton-proton collisions, using jet shape measurements to understand jet quenching effects.

Contribution

It introduces new techniques for background subtraction and two-dimensional unfolding to accurately measure jet shapes in heavy-ion collisions.

Findings

Jet shapes indicate in-medium fragmentation is harder and more collimated.

Modified jet shapes are consistent with jet quenching models.

Results are validated against PYTHIA simulations and proton-proton data.

Abstract

The aim of this work is to explore possible medium modifications to the substructure of inclusive charged jets in Pb-Pb relative to proton-proton collisions by measuring a set of jet shapes. The set of shapes includes the radial moment, $g$, and the momentum dispersion $p_{\mathrm{T}}$D. They provide complementary information on the fragmentation and can help to discriminate between two different scenarios: intra-jet broadening or collimation as a result of jet quenching. \noindent The shapes are measured in Pb--Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV with a constituent cutoff of 0.15 GeV/c and jet resolution $R=$ 0.2. New techniques for background subtraction are applied and a two-dimensional unfolding is performed to correct the shapes to particle level. The corrected jet shapes for jet $p_{\mathrm{T}}$ $40\le p_{\mathrm{T,jet}} \le 60$ GeV/c are presented and discussed. The observed jet shape modifications suggest that the in-medium fragmentation is harder and more collimated than vacuum fragmentation as obtained by a PYTHIA calculation. The PYTHIA calculation is validated with proton-proton data at 7 TeV.