Understanding partonic energy loss from measured light charged particles and jets in PbPb collisions at LHC energies

TL;DR

This study models partonic energy loss in PbPb collisions at LHC energies using nuclear modification factors of charged particles and jets, revealing linear and constant regimes of energy loss across different transverse momentum ranges.

Contribution

It introduces a novel method to quantify partonic energy loss using a piecewise power law fit to nuclear modification factors across multiple collision centralities.

Findings

Energy loss increases linearly with $p_T$ below 5-6 GeV/c.

Energy loss approaches a constant above 22-29 GeV/c.

Jets exhibit approximately linear energy loss even at very high $p_T$.

Abstract

We perform a comprehensive study of partonic energy loss reflected in the nuclear modification factors of charged particles and jets measured in PbPb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 and 5.02 TeV in wide transverse momentum ($p_{\rm T}$) and centrality range. The $p_{\rm T}$ distributions in pp collisions are fitted with a modified power law and the nuclear modification factor in PbPb collisions can be obtained using effective shift ($\Delta p_{\rm T}$) in the spectrum measured at different centralities. Driven by physics consideration, the functional form of energy loss given by $\Delta p_{\rm T}$ can be assumed as power law with different power indices in three different $p_{\rm T}$ regions. The power indices and the boundaries of three $p_{\rm T}$ regions are obtained by fitting the measured nuclear modification factor as a function of $p_{\rm T}$ in all collision centralities simultaneously. The energy loss in different collisions centralities are described in terms of fractional power of number of participants. It is demanded that the power law functions in three $p_T$ regions and their derivatives are continuous at the $p_{\rm T}$ boundaries. The $\Delta p_{\rm T}$ for light charged particles is found to increase linearly with $p_{\rm T}$ in low $p_{\rm T}$ region below $\sim 5-6$ GeV/$c$ and approaches a constant value in high $p_{\rm T}$ region above $\sim 22-29$ GeV/$c$ with an intermediate power law connecting the two regions. The method is also used for jets and it is found that for jets, the $\Delta p_{\rm T}$ increases approximately linearly even at very high $p_{\rm T}$.