Abstract
The yield of 1 < M < 2.5 GeV / c 2 dimuons produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, dimuons from Drell-Yan and from the decays of D meson pairs. These sources properly account for the dimuons produced in p-A collisions. We show that this excess in 158 A GeV In-In collisions is consists of prompt muon pairs. Furthermore, the transverse mass distribution of the excess is well described by an exponential function, with T eff ∼ 190 MeV , independent of mass and significantly lower than the T eff found at M < 1 GeV / c 2 , rising there up to 250 MeV due to radial flow. This suggests the emission of thermal dimuons above 1 GeV / c 2 from a largely partonic source, when radial flow has not yet built up.
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