Measurement of charged-particle spectra in Pb+Pb collisions at $\sqrt{{s}_\mathsf{{NN}}} = 2.76$ TeV with the ATLAS detector at the LHC

TL;DR

This paper reports on measurements of charged-particle spectra in lead-lead and proton-proton collisions at 2.76 TeV using the ATLAS detector, revealing how particle production varies with collision centrality, transverse momentum, and pseudorapidity.

Contribution

It provides detailed nuclear modification factors $R_{AA}$ and $R_{CP}$ as functions of $p_T$, $ ext{eta}$, and centrality, with new insights into their $p_T$-dependence and plateau behavior at high $p_T$.

Findings

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

$R_{AA}$ reaches a plateau above 60 GeV with a value around 0.55.

$R_{AA}$ is consistent with flat $| ext{eta}|$$ dependence across all $p_T$ and centralities.

Abstract

Charged-particle spectra obtained in 0.15 nb${}^{-1}$ of Pb+Pb interactions at $\sqrt{{s}_\mathsf{{NN}}}=2.76$TeV and 4.2 pb${}^{-1}$ of pp interactions at $\sqrt{s}=2.76$ TeV with the ATLAS detector at the LHC are presented in a wide transverse momentum ($0.5 < p_{\mathrm{T}} < 150$ GeV) and pseudorapidity ($|\eta|<2$) range. For Pb+Pb collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeter located on both sides of the interaction point. The nuclear modification factors $R_{\mathrm{AA}}$ and $R_{\mathrm{CP}}$ are presented in detail as function of centrality, $p_{\mathrm{T}}$ and $\eta$. They show a distinct $p_{\mathrm{T}}$-dependence with a pronounced minimum at about 7 GeV. Above 60 GeV, $R_{\mathrm{AA}}$ is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is $0.55\pm0.01(stat.)\pm0.04(syst.)$ in the most central collisions. The $R_{\mathrm{AA}}$ distribution is consistent with flat $|\eta|$ dependence over the whole transverse momentum range in all centrality classes.