First measurement of the rho spectral function in nuclear collisions

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158 A GeV In - In collisions. A strong excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size close to 400 000 events and the good mass resolution of about 2% made it possible to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum shows some non-trivial centrality dependence, but is largely consistent with a dominant contribution from pi(+) pi(-) -> rho -> mu(+) mu(-) annihilation. The associated rho spectral function exhibits considerable broadening, but essentially no shift in mass. The p(T)-differential mass spectra show the excess to be much stronger at low pT than at high pT. The results are compared to theoretical model predictions; they tend to rule out models linking hadron masses directly to the chiral condensate.