On the Nuclear Capture of Muons with Electron Emission

Two experiments carried out to search for the process of muon capture with electron emission are reported. The second of the two experiments is nearly 200 times more sensitive than earlier attempts to find this capture mode, but no indication is obtained in favor of the latter. In both experiments negative muons are made to stop in copper, where coherent capture is predominant, so that the "capture electrons" should be emitted with an energy spectrum sharply peaked around 100 Mev. For the branching ratio of the process searched for, relative to ordinary muon capture, upper limits of about 5×10^-5 and 5×10^-6 are established through the first and second experiment, respectively.