Elliptic flow of thermal photons from event-by-event hydrodynamic model

Understand

Elliptic flow of direct photons in relativistic heavy ion collisions is believed to be dominated by contribution from thermal radiation of quark gluon plasma up to $p_T$ $\sim 5$ GeV/$c$, although other sources start outshining the thermal contribution at already smaller values of $p_T$ in the direct photon spectrum.

• The elliptic flow of thermal photons from ideal hydrodynamics considering a smooth initial density distribution under-predicts the PHENIX direct photon data from 200A GeV Au+Au collisions at RHIC by a large margin in the range $1 < p_T < 5$ GeV/$c$.
• However, a significant enhancement of thermal photon production due to fluctuations in the initial QCD matter density distributions is expected.
• We show that such fluctuations result in substantially larger photon elliptic flow for $p_T > 2.5 $ GeV/$c$ compared to a smooth initial-state-averaged density profile.