Observation of Associated Near-Side and Away-Side Long-Range Correlations in √s(NN)=5.02  TeV Proton-Lead Collisions with the ATLAS Detector

Two-particle correlations in relative azimuthal angle () and pseudorapidity () are measured in √s(NN)= 5.02 TeV p þ Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1 b1 of data as a function of transverse momentum (pT) and the transverse energy (EPb T ) summed over 3:1<<4:9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2 < jj < 5) ‘‘near-side’’ ( 0) correlation that grows rapidly with increasing EPb T . A long-range ‘‘away-side’’ ( ) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small EPb T , is found to match the near-side correlation in magnitude, shape (in and ) and EPb T dependence. The resultant correlation is approximately symmetric about =2, and is consistent with a dominant cos2 modulation for all EPb T ranges and particle pT.