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 $\sqrt{s}_{NN} = 5.02$ TeV. A total integrated luminosity of 30 nb$^{−1}$ was collected during LHC proton-lead runs in the fall and winter of 2012-13. The centrality of proton-lead collisions was characterized using the total transverse energy ($\sum E_{T}^{Pb}$) measured in the ATLAS forward calorimeter on the Pb-going side of the detector. Two analyses on two and multi-particle correlations have been performed using different techniques on the first pilot run of proton-lead collisions (L $\approx$ 1 μb$^{−1}$) taken with a minimum bias trigger. In the first analysis, two-particle correlations in relative azimuthal angle ($\Delta \phi$) and pseudorapidity ($\Delta \eta$) are measured. The correlation function, constructed from charged particle pairs, shows a long-range (2 < $|\Delta \eta |$ < 5) near-side ($\Delta \phi \approx 0$) correlation that grows rapidly with increasing $\sum E_{T}^{Pb}$. After subtracting the recoil correlations due to dijets and low pT resonance decays, a long-range (2 <$|\Delta \eta |$ < 5) away-side ($\Delta \phi \approx \pi$) correlation is found with characteristics matching the near-side correlation in magnitude, shape, pT and $\sum E_{T}^{Pb}$ dependence. The fourier analysis of the $\Delta \phi$ correlation difference between central and peripheral events allowed to extract the elliptic flow coefficients $v_2$ (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 on two particle and four particle combinations. Results are shown as a function of pT and $\sum E_{T}^{Pb}$. The measurements of four particle combinations, sensitive to flow effects only, and the two particle correlations (recoil subtracted) are in good agreement. The main results are compared with the few theoretical models available for p+Pb interactions in the TeV energy regime.