We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360 000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from π+π-→ρ→μ+μ- annihilation. The associated space-time averaged ρ spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.
Physical Review Letters
- Pub Date:
- April 2006
- Relativistic heavy-ion collisions;
- Quark-gluon plasma;
- Inclusive production with identified leptons photons or other nonhadronic particles;
- Nuclear Experiment