Quantifying the Prompt Intermediate Mass Dimuon Excess in the In-In Collisions Measured by NA60

We present improved, albeit still preliminary, results on the production of intermediate mass dimuons in 158 A⋅GeV In-In collisions at the CERN SPS. The NA38 and NA50 experiments observed a significant excess in this region with respect to the dimuon production rate expected from p-A data. Thanks to the use of a pixel vertex tracker, NA60 can separate prompt dimuons from muon pairs resulting from open charm decays and show that the excess dimuons are of prompt origin. The present results are taken from a major reanalysis of the data, where the alignment of the almost one million channels of the radiation-tolerant silicon pixel vertex tracker is now better than 5 μm, significantly improving the discriminating power between prompt dimuons and displaced muon pairs.