Measurement of the charged-hadron spectra and nuclear modification factor in lead--lead collisions with the ATLAS detector

TL;DR

This paper reports measurements of charged-hadron spectra and the nuclear modification factor in lead-lead collisions at the LHC, revealing a characteristic $p_T$ dependence and centrality effects, crucial for understanding quark-gluon plasma properties.

Contribution

First detailed measurement of charged-hadron spectra and nuclear modification factor in Pb+Pb collisions at the LHC across wide $p_T$ and $ eta$ ranges, with comparison to $pp$ data.

Findings

$R_{AA}$ shows a minimum at about 7 GeV

$R_{AA}$ is approximately 0.55 in central collisions at high $p_T$

$R_{AA}$ exhibits flat $ eta$ dependence across all $p_T$ and centralities

Abstract

The ATLAS experiment at the Large Hadron Collider (LHC) measures charged hadron spectra obtained in 2010 and 2011 lead-lead LHC data taking periods with total integrated statistics of 0.15nb${}^{-1}$. The results are compared to the $pp$ spectra of charged hadrons at the same centre-of-mass energy based on the data sample with integrated luminosity of 4.2pb${}^{-1}$ obtained by the ATLAS experiment in 2011 and 2013. This allows a detailed comparison of the two collision systems in a wide transverse momentum ($0.5<p_{\mathrm{T}}<150$GeV) and pseudorapidity ($|\eta|<2$) ranges in different centrality intervals of Pb+Pb collision. The nuclear modification factor $R_{\mathrm{AA}}$ is presented in detail as a function of centrality, $p_{\mathrm{T}}$ and $\eta$. It shows a distinct $p_{\mathrm{T}}$-dependence with a pronounced minimum at about 7GeV. Above 60GeV, it is consistent with a flat, centrality-dependent, value within the uncertainties. The value is $0.55\pm0.01(stat.)\pm0.04(syst.)$ in the most central collisions. The $R_{\mathrm{AA}}$ is observed to be consistent with flat $|\eta|$ dependence over the whole transverse momentum range in all centrality classes.