Medium-induced optical effects for prompt photons

Electromagnetic aspects of a QCD matter have been a hot topic in recent years. High-energy heavy-ion experiments revealed that flow harmonics of direct photons are not well-explained by most hydrodynamic models. In this work, I discuss possible effects of refraction by the hot medium since it can work as a lens which converts its geometrical anisotropy into momentum anisotropy of photons. Flow harmonics of prompt photons are estimated numerically. The results indicate that the effect of refraction is small but phenomenologically non-trivial, exhibiting sensitivity to the QCD refractive index.