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The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c^2 producedin heavy-ion collisions significantly exceeds the sum of the two expectedcontributions, Drell-Yan dimuons and muon pairs from the decays of D mesonpairs. These sources properly account for the dimuons produced inproton-nucleus collisions. In this paper, we show that dimuons are alsoproduced in excess in 158 A GeV In-In collisions. We furthermore observe, bytagging the dimuon vertices, that this excess is not due to enhanced D mesonproduction, but made of prompt muon pairs, as expected from a source of thermaldimuons specific to high-energy nucleus-nucleus collisions. The yield of thisexcess increases significantly from peripheral to central collisions, both withrespect to the Drell-Yan yield and to the number of nucleons participating inthe collisions. Furthermore, the transverse mass distributions of the excessdimuons are well described by an exponential function, with inverse slopevalues around 190 MeV. The values are independent of mass and significantlylower than those found at masses below 1 GeV/c^2, rising there up to 250 MeVdue to radial flow. This suggests the emission source of thermal dimuons above1 GeV/c^2 to be of largely partonic origin, when radial flow has not yet builtup.

Evidence for the production of thermal muon pairs with masses above 1 GeV/c^2 in 158A GeV Indium-Indium Collisions