Photonic measurements of the longitudinal expansion dynamics in relativistic heavy-ion collisions

Owing to the smallness of the electromagnetic coupling, photons escape from the hot and dense matter created in a heavy-ion collision at all times, in contrast to hadrons, which are predominantly emitted in the final freeze-out phase of the evolving system. Thus, the thermal photon yield carries an imprint from the early evolution. We suggest how this fact can be used to gain information on where the actual evolution can be found between the two limiting cases of Bjørken (boost-invariant expansion) and Landau (complete initial stopping and re-expansion) hydrodynamics. We argue that both the rapidity dependence of the photon yield and photonic two-particle correlation radii are capable of answering this question.