Principal Component Analysis of elliptic flow fluctuations in PbPb collisions at ALICE

Abstract

Event-by-event flow fluctuations have long been a recognized influence on anisotropic flow measurements and the cause of factorization breaking of two-particle correlations. A Principle Component Analysis (PCA) provides a measure for anisotropic flow and quantifies these underlying fluctuations. PCA produces leading modes, which are comparable with flow harmonics (v_{n}), and sub-leading modes, which relate to different causes of event-by-event flow fluctuations. PCA of elliptic flow and its fluctuations is presented at sqrt{s_{NN}}=2.76 TeV in PbPb collisions for two-particle azimuthal correlations as a function of pseudorapidity (eta) and transverse momentum (p_{T}) with data from the ALICE detector at the LHC. The results are presented for multiple centrality windows. The leading modes and the first sub-leading modes are used to present factorization breaking effects for both p_{T}- and eta-dependence. Also a comparison is made between PCA and the accepted Q-cumulants method for two-particle correlations. It can be concluded that PCA can be used to determine elliptic flow and, moreover, gives new information about event-by-event effects on flow. These effects can be attributed to hydrodynamic processes, namely the event-by-event initial density fluctuations and torque effects due to forward-backward moving particles.