Thermal Dileptons from Hot and Dense Strongly Interacting Matter

Understand

The NA60 experiment at the CERN SPS has studied muon-pair production in 158A GeV In-In collisions.

• The unprecedented precision of the data has allowed to isolate a strong excess of pairs above the known sources in the whole invariant mass region 0.2<M<2.6 GeV.
• The (mostly) Planck-like shape of the mass spectra, exponential m_T spectra, zero polarization and the general agreement with thermal-model results allow for a consistent interpretation of the excess dimuons as thermal radiation from a randomized system.
• For M<1 GeV, the process pi+pi- -> rho -> mu+mu- dominates.