Anisotropy of Low Energy Direct Photons in Relativistic Heavy Ion   Collisions
  We investigate the behavior of low energy photons radiated by the deceleration processes of two colliding nuclei in relativistic heavy ion collisions using the Wigner function approach for electromagnetic radiation fields. The angular distribution reveals the information of the initial geometric configurations. Such a property is reflected in the anisotropic parameter $v_{2}$, showing an increasing $v_{2}$ as energy decreases, which is a behavior qualitatively different from $v_{2}$ from hadrons produced in the collisions. 
T. Koide and T. Kodama
nucl-th hep-ex hep-ph