Observation of long-range elliptic anisotropies in [formula omitted] and 2.76 TeV pp collisions with the ATLAS detector

Abstract

ATLAS has measured two-particle angular correlations in s=13 and 2.76 TeV pp collisions at the LHC. The well-known “ridge” is observed in high-multiplicity events. Per-trigger-particle yields, Y(Δϕ), are found to be consistent with a linear combination of the per-trigger-yield measured with <20 reconstructed tracks, and a constant combinatoric contribution modulated by cos⁡(2Δϕ). The fitted Fourier coefficient v2,2 exhibits factorization, suggesting that the ridge results from per-event cos⁡(2ϕ) modulation of the single-particle distribution with Fourier coefficient v2. They are found to be weakly dependent on multiplicity and to have a pT dependence similar to that measured in p+Pb and Pb+Pb collisions. The v2 values in the 13 and 2.76 TeV are consistent within uncertainties. These results suggest that the ridge in pp collisions arises from the same or similar underlying physics as observed in p+Pb collisions, and that the dynamics responsible for the ridge has no strong s dependence.