Prompt and displaced muon production in Indium-Indium collisions as measured by the NA60 experiment

This work addresses the production of intermediate mass muon pairs in indium-indium collisions, at a beam momentum of 158 GeV per nucleon, as measured by the NA60 experiment at the CERN SPS. A radiation-tolerant silicon pixel tracker measures the trajectories of charged particles at the interaction level. In order to explore the measurement of single muon offsets with respect to the interaction vertex, we have developed a background subtraction method that deals with the vertex dipole magnetic field and wrong matches between muons and vertex tracks. We then compare the data in the region between the f and J/y resonances to a superposition of muon pairs from simultaneous semi-leptonic decays of D mesons and Drell-Yan prompt dimuons. Doing this in the dimuon mass dimension provides results compatible with those previously reported by the NA38 and NA50 experiments. Performing the same analysis using the unique ability of NA60 to measure the muon offsets, we conclude that the excess is prompt in nature, has a mass shape steeper than Drell-Yan and grows faster than linearly with the number of nucleons participating in the interaction.