On the Penetrating Nature of Electromagnetic Probes
  We establish the extent to which photons and dileptons are penetrating probes of hot hadronic matter by computing mean free paths. Absorption is indeed negligible for real photons as mean free paths are roughly tenths of \AAngstr\"oms. However, vector dominance effects, which provide rich structure in virtual photon mass spectra, introduce nonnegligible reabsorption and rehadronization resulting in mean free paths of the order of tens of femtometers. Consequences are illustrated by considering the net dilepton yield near the rho peak via thermal \pi\pi annihilation from a short-lived hadronic fireball. Results suggest that disappearance of the rho in heavy-ion collisions can be due in part to reabsorption/rehadronization rather than solely due to collective effects. 
Kevin L. Haglin