Non-collective component in v2 and how to detect it

Current RHIC data on elliptic flow v2 as a function of pT display dinstinctive patterns at low and high pT due to different sources of particle yield: the hydrodynamic collective flow dominating the low pT and the non-collective hard processes dominating the high pT. The intermediate region where v2 shows nontrival structure is yet not well understood. We emphasize the non-collective source which can also contribute to the observed v2 at low to intermediate pT. We further show that the backward-forward elliptic anisotropy correlation provides an experimentally accessible observable which distinguishes between collective and non-collective contributions to the observed v2. The measurement of this observable will reveal valuable information on the interpolation pattern at intermediate pT between the low-pT collective flow regime and the high-pT (semi)-hard processes regime. We also argue that the shift of dominance between the collective and non-collective sources is an alternative explanation to the dropping v2 at intermediate pT which may have important impact on quantifying the shear viscosity from v2 data. Reference: Jinfeng Liao \& Volker Koch, arXiv: 0902.2377.