Measurement of two–particle correlations in √s = 13 TeV proton–proton collisions at the LHC with the ATLAS detector

ATLAS measurements of two-particle correlations in √s = 13 TeV pp collisions at the LHC are presented. The correlations are measured using charged particles detected over the pseudorapidity region |η| < 2.5, and are presented as a function of azimuthal-angle (∆φ) and pseudorapidity (∆η) separation for different intervals of measured charged-particle mul- tiplicity. In events with a large multiplicity of reconstructed charged-particle tracks, an enhancement is observed in the two-particle correlation function at ∆φ = 0 that extends over a wide range of ∆η. This feature, often referred to as the “ridge”, is similar to that seen in previous pp, p+Pb, and Pb+Pb measurements. One-dimensional correlation functions integrated over 2 ≤ |∆η| < 5 and corresponding per-trigger-particle yields are presented and compared to similar correlations obtained from Pythia 8, which show no ridge. Per-trigger- particle yields integrated over ∆φ are estimated using the “zero yield at minimum” method. They are observed to grow rapidly with charged-particle multiplicity at large multiplicity. They also increase with charged-particle transverse momentum reaching a maximum near pT = 2.5 GeV and then decrease at higher pT. A comparison of the per-trigger-particle yields to those previously measured by the CMS collaboration in pp collisions at √s = 7 TeV shows similar yield for events having the same multiplicity.