First measurement of the ρ spectral function in nuclear collisions

Abstract

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 π + π − → ρ → μ + μ − annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass.