Global characteristics of the proton-lead collisions in ATLAS

The ATLAS experiment has studied the centrality dependence of global event characteristics in proton-lead collisions at cms energies of 6,02 TeV. A total integrated luminosity of 30 inv nb was collected during LHC proton-lead runs in the fall of 2012 and winter of 2013. Two analyses on two and multi-particle correlations have been performed using different techniques on the first pilot run of proton-lead collisions (with integrated luminosity L =1 inv mb) taken with a minimum bias trigger. In both analyses, the centrality of proton-lead collisions was characterized using the total transverse energy (sum EPb T ) measured in the ATLAS forward calorimeter on the Pb-going side of the detector. In the first analysis, two-particle correlations in relative azimuthal angle (Df) and pseudorapidity (Dh) are measured. The correlation function, constructed from charged particle pairs, shows a long-range (2 < jDhj < 5) near-side correlation that grows rapidly with increasing sum EPb T . After subtracting the recoil correlations due to dijets and low pT resonance decays, a long-range (2 < jDhj < 5) away-side (Df p) correlation is found with characteristics matching the near-side correlation in magnitude, shape, pT and åEPb T dependence. The Fourier analysis of the Df correlation with the subtracted recoil component allowed to extract the elliptic flow coefficients v2(pT) as a function of pT of one of the particles forming the pair. In a second analysis on the same data, the elliptic flow coefficients have been extracted with the cumulant method for two particles and four particles. Results are shown as a function of pT and åEPb T . The elliptic flow measurements on four particles, v2f4g, sensitive to flow effects only, and on two particle correlations (recoil subtracted), v2f2PCg, are in good agreement. The main results are compared with the few theoretical models available for p+Pb interactions in the TeV energy regime.