Low Mass Dimuon Production in Indium-Indium Collisions at the CERN SPS

NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus-nucleus and proton-nucleus collisions. In this paper we report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions. A significant excess of pairs is observed above the yield expected from neutral meson decays. The excess can be isolated by subtraction of expected sources, thanks to the excellent mass resolution and large sample size.Comment: 4 pages, 3 figures, Contribution at XLIst Rencontres de Moriond, "QCD and High Energy Hadronic Interactions

Thermal Dimuon Emission in In-In at the CERN SPS

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. A significant excess of pairs is observed above the yield expected from neutral meson decays, consistent with a dominant contribution from $\pi\pi\to\rho\to\mu\mu$. This paper presents precision results on the mass and transverse momentum spectra of the excess pairs. The space-time averaged rho spectral function associated to the measured mass distribution shows a significant broadening, but essentially no mass shift. The slope parameter $T_{eff}$ extracted from the spectra rises with dimuon mass up to the $\rho$, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline indicates a transition to an emission source with much smaller flow, possibly of partonic origin.The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. A significant excess of pairs is observed above the yield expected from neutral meson decays, consistent with a dominant contribution from $\pi$ $\pi$ $\to$ $\rho$ $\to$ $\mu$ $\mu$.This paper presents precision results on the mass and transverse momentum spectra of the excess pairs. The space-time averaged rho spectral function associated to the measured mass distribution shows a significant broadening, but essentially no mass shift. The slope parameter $T_{eff}$ extracted from the spectra rises with dimuon mass up to the $\rho$ , followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline indicates a transition to an emission source with much smaller flow , possibly of partonic origin