Energy dependence of the freeze out eccentricity from the azimuthal dependence of HBT at STAR

Abstract

Non-central heavy ion collisions create an out-of-plane-extended participant zone that expands toward a more round state as the system evolves. The recent RHIC Beam Energy Scan at sqrt{s_{NN}} of 7.7, 11.5, and 39 GeV provide an opportunity to explore the energy dependence of the freeze out eccentricity. The new low energy data from STAR complements high statistics data sets at sqrt{s_{NN}} of 62.4 and 200 GeV. Hanbury-Brown-Twiss (HBT) interferometry allows to determine the size of pion emitting source regions. The dependence of the HBT radius parameters on azimuthal angle relative to the reaction plane have been extracted. These dependencies can be related to the freeze out eccentricity. The new results from STAR are consistent with a monotonically decreasing freeze out eccentricity and constrain any minimum, suggested by previously available data, to lie in the range between 11.5 and 39 GeV. Of several models UrQMD appears to best predict the STAR and AGS data.