Anomalous muon production in e+e- annihilations as evidence for heavy leptons

We have measured inclusive muon production in e⁺e^- annihilation for CMS energies between 3.6 and 5.0 GeV. Above 4 GeV the cross section cannot be explained by conventional sources like higher order QED processes or inclusive production of the J/ϑ (3.1). It is, however, compatible with the pair production of heavy particles of a mass of about 1.9 GeV/c². Spin assignment and decay parameters are investigated.