Photon and dilepton observables: An experimental overview

Understand

Dilepton and direct photon measurements in heavy ion collisions provide access to the early stages of the collision.

• Dilepton continuum measurements at SPS and RHIC show an excess consistent with broadening of the rho meson spectral function as a result of hadronic interactions.
• Direct photon measurements at RHIC and the LHC show an excess of low $p_{T}$ photons over the expected yield.
• This excess has a large azimuthal anisotropy with respect to the collision plane.