1 Is there elliptic flow without transverse flow?

Abstract

Azimuthal anisotropy of final particle distributions was originally introduced as a signature of transverse collective flow [1]. We show that finite anisotropy in momentum space can result solely from the shape of the particle emitting source. However, by comparing the differential anisotropy to recent data from STAR collaboration [2,3] we can exclude such a scenario, but instead show that the data favour strong flow as resulting from a hydrodynamical evolution [4,5]. 1. Bjorken Brick The intensity of particles emitted from a steady thermal source depends on the size of the source facing the direction of emitted particles. If the shape of such a source is not azimuthally symmetric, the particle yield is anisotropic too. Thus it is easy to parametrize a source with an essentially arbitrary value of the elliptic flow coefficient v2, but the transverse momentum dependence of the differential v2(pt) is much less trivial. To gain insight what kind of elliptic flow might result from surface effects only, we constructed a simple parametrization of the source. Let us assume that the longitudinal expansion of the source is boost invariant, but that the source does not expand transversally