Long-range correlations in proton-lead collisions at sqrt{s_NN} = 5.02 TeV from ATLAS

Two-particle correlations in relative azimuth $\Delta\phi$ and relative pseudorapidity $\Delta\eta$ are studied in p+Pb collisions at sqrt{s_NN}=5.02 TeV with the ATLAS detector at LHC. The correlations are studied as a function of charged particle pT and the collision E_T^fcal summed over 3.1 < eta < 4.9 in the direction of the Pb beam. After subtracting the known sources of correlations such as dijets, resonances and momentum conservation, estimated using events with low E_T^fcal, the resulting correlations exhibit a $\Delta\phi$ modulation that is flat in $\Delta\eta$ out to $|\Delta\eta|=5$. The modulation is comparable in magnitude to similar modulations observed in heavy ion collisions, and can be described by a $1+2c_2\cos2\Delta\phi+2c_3\cos3\Delta\phi$ function over 0.5<pT<7 GeV in broad ranges of E_T^fcal. The correlation analysis is repeated for event classes defined by the number of reconstructed charged particles N_ch^rec. This analysis gives nearly the same result as the analysis based on E_T^fcal for the long-range correlation at the near-side ($\Delta\phi~0$), but leads to biases in the long-range correlations at the away-side ($\Delta\phi\sim\pi$). HIJING simulation suggests that this bias is mainly associated with the contributions from dijets which are correlated strongly with the N_ch^rec.