Dimuon production in 15.5-GeV/c $\pi p$ interactions and the observation of a low-mass continuum

We report the results of an experiment which examines the entire final state of events in which muon pairs were produced in 15.5-GeV/c ${\pi }^{\pm }p$ interactions. A portion of this data was presented earlier. Here we report on the final data sample of 890 events obtained from a total of 2.5×${10}^8$ interactions [a sensitivity of (8.65 events/nb)×trigger efficiency]. The experiment was sensitive primarily to the low-mass region and the good mass resolution made it possible to identify the contributions from the $\rho$ and $\omega$ decays and the $\eta$ and $\omega$ Dalitz decays. An excess of events is observed in the dimuon mass region ∼400 - 600 MeV which is not accounted for by the above-mentioned decays. The final states were examined to search for characteristic differences from those in events without dimuons.