| Experimental measurements of the Michel parameter h can be used, along with the other Michel parameters appearing in the description of muon decays, to set limits on possible violations of the V - A form of the weak interaction. All of the Michel parameters, save for h , can be measured by analyzing the ordinary ration decay m+→e+ne nm . To measure h , the radiative decay m+→e+ne nmg must be observed. This work is based on more than 4 x 10 5 radiative muon decays observed at the Paul Scherrer Institute meson factory using a large acceptance spectrometer. Based on these events we measure the branching ratio for the radiative decay, with the restrictions Egamma > 10 MeV on the photon energy and theta > 30° on the positron/photon opening angle, to be B = [4.40+/-0.02 (stat.)+/-0.09 (syst.)] x 10-3. The best fit for the branching ratio is found to occur for h , = -0.084 +/- 0.050(stat.) +/- 0.034(syst.), to be compared to the V - A Standard Model value h SM = 0. We interpret our result as an upper limit on the allowed value: h ≤ 0.033 (68% confidence). Combined with other measurements of h , this reduces the known upper limit to h ≤ 0.028 (68% confidence). |
