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

The ATLAS experiment at the Large Hadron Collider (LHC) measures charged hadron spectra obtained in 2010-2011 lead-lead LHC data taking periods with total integrated statistics of 0.15 nb${}^{-1}$. 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 pb${}^{-1}$ obtained by the ATLAS experiment in 2013. These allows for 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 factors $R_\mathrm{AA}$ and $R_\mathrm{CP}$ are presented in detail as a function of centrality, $p_\mathrm{T}$ and $\eta$. They show a distinct $p_\mathrm{T}$-dependence with a pronounced minimum at about 7GeV. Above 60GeV, $R_\mathrm{AA}$ is consistent with a flat, centrality-dependent, value within the uncertainties. The value is 0.55±0.01(stat.)±0.04(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.