Electromagnetic Emission Rates and Spectral Sum Rules
  The electromagnetic emission rates at SPS energies satisfy spectral constraints in leading order in the pion and nucleon densities. These constraints follow from the strictures of broken chiral symmetry. We saturate these constraints using available data, leading to model independent emission rates from a hadronic gas. With a simple fire-ball scenario, only large nucleon densities may account for the present CERES data. 
James Steele, Hidenaga Yamagishi, and Ismail Zahed