Evidence for collectivity in pp collisions at the LHC

Measurements of twoand 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 (v2) and third-order (v3) azimuthal anisotropy harmonics of unidentified charged particles, as well as v2 of KS and Λ/Λ 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), KS, and Λ/Λ, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pT ≈ 2 GeV/c. For 13 TeV data, the v2 signals are also extracted from fourand six-particle correlations for the first time in pp collisions, with comparable magnitude to those from twoparticle correlations. These observations are similar to those seen in pPb and PbPb collisions, and support the interpretation of a collective origin for the observed longrange correlations in high-multiplicity pp collisions. Published in Physics Letters B as doi:10.1016/j.physletb.2016.12.009. c © 2016 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license ∗See Appendix A for the list of collaboration members ar X iv :1 60 6. 06 19 8v 2 [ nu cl -e x] 2 4 D ec 2 01 6