Measurements of charged particle spectra and nuclear modification factors in p+Pb and Pb+Pb collisions with the ATLAS detector

The measurement of charged particle spectra in heavy ion collisions carries important information about the properties of hot and dense matter created in these interactions. Spectra measured in lead-lead collisions at different centralities can be compared to the proton-proton spectra giving quantitative information about the properties of such matter. Proton-nucleus collisions provide further means for understanding the role of the initial state effects modifying the hard scattering rates. The ATLAS detector at the LHC obtained the sample of Pb+Pb data at $\sqrt{s_{NN}}$= 2.76 TeV with integrated luminosity 0.15 nb$^{-1}$, which can be compared to recently obtained pp sample of 4.5 pb$^{-1}$ sample at the same energy. The p+Pb data at $\sqrt{s_{NN}}$= 5.02 TeV with integrated luminosity 30 nb$^{-1}$ can also be compared to the pp data obtained by interpolating pp measurements at $\sqrt{s}$= 2.76 TeV and 7 TeV. Due to the excellent capabilities of the ATLAS detector, and its stable operation in heavy ion as well as proton-proton physics runs, the data allow measurements of the nuclear modification factor out to transverse momentum limited only by the statistics of the accumulated samples and studying the ratios of HI charged particle spectra divided by pp reference in different centrality bins over a wide range of pseudorapidity.