Soft probes of the QGP: Pb-Pb and p-Pb CMS results

Abstract

The CMS collaboration measured second-order single-particle anisotropy, v(2), using 2-, 4-, 6- and 8-particle correlations as well as the Lee-Yang Zero method in pPb and PbPb collisions. This result shows that the correlations found in the small and highly asymmetric pPb collision system are long-range and have a many-particle origin. Deeper insight into the nature of these correlations is achieved by studying the factorization breaking effect which appears trough a PT-dependent event plane angle, caused by the initial-state fluctuations. The effect is largest for the most central PbPb collisions (up to 20%), while it is on the level of few percent for peripheral PbPb and high-multiplicity pPb events. Hydrodynamics models which include a PT-dependent event plane angle semi-quantitatively describe the data and suggest that the effect is mainly determined by the initial-state conditions.