Observation of Long-Range Elliptic Azimuthal Anisotropies ins=13and 2.76 TeVppCollisions with the ATLAS Detector

Abstract

ATLAS has measured two-particle correlations as a function of the relative azimuthal angle, Δϕ, and pseudorapidity, Δη, in ffiffi ffi s p ¼ 13 and 2.76 TeV pp collisions at the LHC using charged particles measured in the pseudorapidity interval jηj < 2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at Δϕ ∼ 0 that extends over a wide range of Δη, which has been referred to as the "ridge." Per-trigger-particle yields, YðΔϕÞ, are measured over 2 < jΔηj < 5. For both collision energies, the YðΔϕÞ distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by cos ð2ΔϕÞ. The fitted Fourier coefficient, v 2;2 , exhibits factorization, suggesting that the ridge results from per-event cos ð2ϕÞ modulation of the single-particle distribution with Fourier coefficients v 2 . The v 2 values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a p T dependence similar to that measured in p þ Pb and Pb þ Pb collisions. The v 2 values in the 13 and 2.76 TeV data 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 þ Pb collisions, and that the dynamics responsible for the ridge has no strong ffiffi ffi s p dependence. DOI: 10.1103/PhysRevLett.116.172301 Measurements of two-particle angular correlations in high-multiplicity proton-proton (pp) collisions at a centerof-mass energy ffiffi ffi s p ¼ 7 TeV at the LHC showed an enhancement in the production of pairs at small azimuthal-angle separation, Δϕ, that extends over a wide range of pseudorapidity differences, Δη, and which is often referred to as the "ridge" [1]. The ridge has also been observed in proton-lead (p þ Pb) collisions [2][3][4][5][6][7], where it is found to result from a global sinusoidal modulation of the per-event single-particle azimuthal angle distributions [3][4][5][6] TeV data recorded during LHC run 2 and run 1, respectively, to address these issues. The maximum number of inelastic interactions per crossing was 0.04 and 0.5 for the 13 and 2.76 TeV data, respectively. Two-particle angular correlations are measured as a function of Δη and Δϕ in different intervals of the measured charged-particle multiplicity and different p T intervals spanning 0.3 < p T < 5 GeV: 0.3-0.5 GeV, 0.5-1 GeV, 1-2 GeV, 2-3 GeV, 3-5 GeV. Separate p T -integrated results use 0.5 < p T < 5 GeV. Per-trigger-particle yields are obtained from the long-range (jΔηj > 2) component of the correlation. A new template-fitting method is applied to these yields to test for sinusoidal modulation similar to that observed in p þ Pb collisions. The measurements were performed using the ATLAS inner detector (ID), min...

Measurements of long-range azimuthal anisotropies and associated Fourier coefficients for pp collisions at s=5.02 and 13 TeV and pAaboud1, Bergeås2, Brenner3 et al. 2017Phys. Rev. CSelf Cite734780View full textAdd to dashboardCite

“…The study of two-particle correlations in this paper follows previous ATLAS measurements in Pb + Pb [13,69,70], p + Pb [2,4], and pp [41] collisions. For a given event class, the primary charged particles is somewhat tighter in the pp simulation [68].…”

“…[41] and provides new measurements of such correlations in pp collisions at √ s = 5.02 TeV. It also presents a reanalysis of two-particle correlations in 5.02 TeV p + Pb collisions and presents a direct comparison between the pp and p + Pb data at the same per-nucleon center-of-mass energy as well as a comparison between the pp data at the two energies.…”

“…Section V describes the template fitting of the two-particle correlations, which was originally developed in Ref. [41]. The template fits are used to extract the Fourier harmonics v n,n [Eq.…”

“…The 13 TeV pp data were recorded over two periods: a set of low-luminosity runs in June 2015 (used in Ref. [41] …”

“…[41]. In this procedure, the measured Y ( φ) distributions are assumed to result from a superposition of a "peripheral"…”