Observation of long-range, near-side angular correlations in pPb collisions at the LHC

Abstract

Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, eta, and full azimuth, phi, as a function of charged particle multiplicity and particle transverse momentum, p(T). In high-multiplicity events, a long-range (2 < vertical bar Delta eta vertical bar < 4), near-side (Delta phi approximate to 0) structure emerges in the two-particle Delta eta-Delta phi correlation functions. This is the first observation of such correlations in proton-nucleus collisions, resembling the ridge-like correlations seen in high-multiplicity pp collisions at root s = 7 TeV and in AA collisions over a broad range of center-of-mass energies. The correlation strength exhibits a pronounced maximum in the range of p(T) = 1-1.5 GeV/c and an approximately linear increase with charged particle multiplicity for high-multiplicity events. These observations are qualitatively similar to those in pp collisions when selecting the same observed particle multiplicity, while the overall strength of the correlations is significantly larger in pPb collisions. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved

Azimuthal anisotropy of charged particles from multiparticle correlations in pPb and PbPb collisions with CMS

“…These long-range correlations can be interpreted as hydrodynamic collective flow of the strongly interacting and expanding medium. However, such long-range correlations have also been reported in proton-proton (pp) [11] and proton-lead (pPb) [12,13,14] collisions with very high final-state particle multiplicity at the LHC. Also, evidence of such correlations was also found in 200 GeV deuteron-gold (dAu) at RHIC [15].…”

Mid-rapidity charged hadron transverse spherocity in pp collisions simulated with Pythia

“…Perhaps the most interesting result is the discovery of sQGP-like (ridge-like and flow-like behavior) features in small systems like those created in pp and p-Pb collisions [1][2][3][4][5][6]. The origin of such effects is still far from being understood, for instance, it has been shown that hydro calculations, where the formation of a hot and dense QCD medium is implicitly assumed, can describe qualitatively many features of data [7][8][9].…”

Measurements of heavy-flavour decay leptons with ALICE

“…In Fig. 7 EPJ Web of Conferences 04058-p. 6 (left), the correlation distribution in high-multiplicity (0-20%) collisions after subtracting the correlation measured in low-multiplicity (60-100%) collisions is shown. Long-range correlations are observed in pseudorapidity on the near-side and away-side in azimuthal direction.…”

“…One of the most interesting results in p-Pb collisions was the observation in high multiplicity events of two long-range ridge-like structures in the distributions of angular correlations between two charged particles after removing correlations due to jets as extracted from low-multiplicity collisions [6][7][8]. This double-ridge structure can be interpreted as due to a collective expansion of the final-state particles or to gluon saturation effects in the initial state in the Colour Glass Condensate (GCG) framework.…”