Measurement of long-range pseudorapidity correlations and azimuthal harmonics in √s_NN =5.02 TeV proton-lead collisions with the ATLAS detector

Measurements of two-particle correlation functions and the first five azimuthal harmonics, v¹ to v⁵, are presented, using 28 nb^-1 of p+Pb collisions at a nucleon-nucleon center-of-mass energy of √sNN=5.02 TeV measured with the ATLAS detector at the LHC. Significant long-range "ridgelike" correlations are observed for pairs with small (|Δϕ|<π/3) and back-to-back pairs (|Δφ|>2π/3) over the transverse momentum range 0.4<p_T<12 GeV and in different intervals of event activity. The event activity is defined by either the number of reconstructed tracks or the total transverse energy on the Pb-fragmentation side. The azimuthal structure of such long-range correlations is Fourier decomposed to obtain the harmonics ν_n as a function of pT and event activity. The extracted ν_n values for n=2 to 5 decrease with n. The ν₂ and ν₃ values are found to be positive in the measured pT range. The ν₁ is also measured as a function of pT and is observed to change sign around pT≈1.5-2.0 GeV and then increase to about 0.1 for pT>4 GeV. The ν₂ (pT), ν₃ (pT), and ν₄ (pT) are compared to the ν_n coefficients in Pb+Pb collisions at √sNN=2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average pT of particles produced in the two collision systems.