NA60 results on thermal dimuons NA60 Collaboration

The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158 A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2 1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differ- ences to Drell-Yan pairs. The slope parameter Teff associ- ated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. The rise for M 1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emis- sion source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck- like shape of the mass spectra at low pT and the lack of