Evidence for collectivity in pp collisions at the LHC

Abstract

Measurements of two- and multi-particle angular correlations in pp collisions at s=5,7, and 13TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0pb?1 (5?TeV), 6.2pb?1 (7?TeV), and 0.7pb?1 (13?TeV), were collected using the CMS detector at the LHC. The second-order (v<inf>2</inf>) and third-order (v<inf>3</inf>) azimuthal anisotropy harmonics of unidentified charged particles, as well as v<inf>2</inf> of K<inf>S</inf> 0 and ?/?? particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the v<inf>2</inf> values of charged hadrons (mostly pions), K<inf>S</inf> 0, and ?/??, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below p<inf>T</inf>?2GeV/c. For 13?TeV data, the v<inf>2</inf> signals are also extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. These observations are similar to those seen in pPb and PbPb collisions, and support the interpretation of a collective origin for the observed long-range correlations in high-multiplicity pp collisions. © 2016 The Author