Evidence for collectivity in pp collisions at the LHC

Abstract

Measurements of two- and multi-particle angular correlations in pp collisions at are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of (5 TeV), (7 TeV), and (13 TeV), were collected using the CMS detector at the LHC. The second-order () and third-order () azimuthal anisotropy harmonics of unidentified charged particles, as well as of 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 values of charged hadrons (mostly pions), , and , with lighter particle species exhibiting a stronger azimuthal anisotropy signal below . For 13 TeV data, the 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.