Evidence for collectivity in pp collisions at the LHC

CMS Collaboration

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Measurements of two- and multi-particle angular correlations in pp collisions at √s=5, 7,and13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0 pb−1 (5TeV), 6.2 pb−1 (7TeV), and 0.7 pb−1 (13TeV), 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 K0S 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), K0S, and Λ/Λ‾, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pT ≈ 2 GeV/c. For 13TeV 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.
Original languageEnglish
Pages (from-to)193-220
Number of pages28
JournalPhysics Letters B
Publication statusPublished - 2017
Externally publishedYes


  • CMS
  • Physics
  • Heavy ion
  • Ridge
  • Correlation
  • pp

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