Evidence for collectivity in pp collisions at the LHC

Abstract

Measurements of two- and multi-particle angular correlations in pp collisions at View the MathML source are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of View the MathML source (5 TeV), View the MathML source (7 TeV), and View the MathML source (13 TeV), were collected using the CMS detector at the LHC. The second-order (v2) and third-order (v3) azimuthal anisotropy harmonics of unidentified charged particles, as well as v2 of View the MathML source and View the MathML source 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 v2 values of charged hadrons (mostly pions), View the MathML source, and View the MathML source, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below View the MathML source. For 13 TeV data, the v2 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.