Evidence for collectivity in pp collisions at the LHC

Measurements of two- and multi-particle angular correlations in pp collisions at √ s = 5, 7, and 13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0 pb^-1 (5 TeV), 6.2 pb^-1 (7 TeV), and 0.7 pb^-1 (13 TeV), were collected using the CMS detector at the LHC. The second-order (ν₂) and third-order (ν₃) azimuthal anisotropy harmonics of unidentified charged particles, as well as ν₂ of K_S⁰ and Λ/Λ-bar 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 ν₂ values of charged hadrons (mostly pions), K_S⁰, and Λ/Λ-bar, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pT approximate to GeV/c. For 13 TeV data, the ν₂ 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 ∼ 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. (authors)