  The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158
AGeV In-In collisions. A strong excess of pairs is observed above the yield
expected from neutral meson decays. The unprecedented sample size close to 400K
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 pT-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.
