First measurement of the ϱ spectral function in nuclear collisions

R. Arnaldi, R. Averbeck, K. Banicz, J. Castor, B. Chaurand, C. Cicalo, A. Colla, P. Cortese, S. Damjanovic, A. David, A. De Falco, A. Devaux (+37 others)
2006 European Physical Journal C: Particles and Fields  
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
more » ... ant 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.
doi:10.1140/epjc/s10052-006-0060-2 fatcat:5juyh4oxrfhqvdx2tbj5duok2a