Measurement of Long-Range Pseudorapidity Correlations and Azimuthal Harmonics in √s[subscript NN] = 5.02 TeV Proton-Lead Collisions with the ATLAS Detector

Abstract

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[subscript 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[subscript n] as a function of p[subscript T] and event activity. The extracted ν[subscript 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 p[subscript T] and is observed to change sign around p[subscript T]≈1.5-2.0 GeV and then increase to about 0.1 for p[subscript T] > 4 GeV. The ν₂ (p[subscript T]), ν₃ (p[subscript T]), and ν₄(p[subscript T]) 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 p[subscript T] of particles produced in the two collision systems.