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

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 p+Pb data at $\sqrt{s_{{}_\mathrm{NN}}}=5.02$ TeV with integrated luminosity 25 nb${}^{-1}$, which can 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, ratios of HI charged particle spectra divided by \pp reference, in different centrality classes over a wide range of rapidity. The charged particle nuclear modification factor is found to vary significantly as a function of transverse momentum with a stronger dependence in more peripheral collisions.