Anisotropy in pPb and PbPb collisions from CMS

Abstract

The recent CMS results on anisotropic particle emission will be presented. A multi-particle nature of the long-range correlations observed in pPb collisions is revealed through consistency between the results obtained using four-, six and eight-particle correlation as well as the Lee-Yang zero method. The magnitude of the elliptic and triangular flow of strange particles from both pPb and PbPb collisions have been extracted by correlating an identified strange hadron (KS0 or Λ/Λ)with a charged particle separated by a large relative pseudorapidity. The results for strange, KS0 and Λ/Λ,particles scaled by the number of constituent quarks plotted as a function of transverse kinetic energy per number of constituent quarks are in a rather good mutual agreement for both v2and v3over a wide range of particle transverse kinetic energy and event multiplicities. The initial-state fluctuations induce that the event-plane angle is not any more a global quantity but depends on both, transverse momenta (pT) and pseudorapidity (η), which further induces the factorization breaking of the two-particle azimuthal anisotropy into a product of single-particle anisotropies. In the pTdirection, maximal effect of factorization breaking of about 20% is observed in ultra-central PbPb collisions. In the η direction, the effect is weakest for mid-central PbPb events and gets larger for more central or peripheral PbPb collisions as well as for high multiplicity pPb collisions. The experimental results are compared with recent hydrodynamic predictions which involve the factorization breakdown effect. The effect is sensitive to the initial-state conditions rather than the shear viscosity of the medium.