The NA60 experiment has studied low-mass muon pairs in 158 AGeV Indium-Indium
collisions at the CERN SPS. A strong excess of pairs is observed above the
expectation from neutral meson decays. The unprecedented sample size of 360 000
events and the good mass resolution of about 2% allow to isolate the excess by
subtraction of the known sources. The shape of the resulting mass spectrum is
consistent with a dominant contribution from pi+ pi- -> rho -> mu+ mu-
annihilation. The associated rho spectral function shows a strong broadening,
but essentially no shift in mass.Comment: 4 pages, 3 figures, Proceedings of Quark Matter 2005, Budapes

Damjanovic, S.

English

arXiv

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.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

First measurement of the rho spectral function in nuclear collisions

Abstract

Similar works

Full text

Available Versions

CERN Document Server