Observation of long-range elliptic anisotropies in $\sqrt{s}=13$ and 2.76 TeV $pp$ collisions with the ATLAS detector

ATLAS has measured two-particle correlations as a function of relative azimuthal-angle, $\Delta\phi$, and pseudorapidity difference, $\Delta\eta$, in $\sqrt{s}=13$ and 2.76 TeV $pp$ collisions at the LHC. The correlation functions show a multiplicity-dependent enhancement at $\Delta\phi\sim0$ that extends a wide range of $\Delta\eta$, known as "ridge". Per-trigger-particle yields, $Y(\Delta\phi)$, 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 $\cos(2\Delta\phi)$. The fitted Fourier coefficient $v_{2,2}$ exhibits factorization, suggesting that the ridge results from per-event $\cos(2\Delta\phi)$ modulation of the single-particle distribution with Fourier coefficient $v_{2}$. They are found to be weakly dependent of multiplicity and to have a $p_{\text{T}}$ dependence similar to that measured in $p$+Pb and Pb+Pb collisions. The $v_{2}$ values in the 13 and 2.76 TeV are consistent within uncertainties. These results suggest that the ridge in $pp$ collisions arise from the same or similar underlying physics as observed in $p$+Pb collisions, and that the dynamics responsible for the ridge has no strong $\sqrt{s}$ dependence.