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 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 (v₂) and third-order (v₃) azimuthal anisotropy harmonics of unidentified charged particles, as well as v₂ of KS₀ and Λ/ Λ̄ 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), KS₀, and Λ/ Λ̄, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pT≈2GeV/c. 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.