Measurement of the energy spectrum of νe from muon decay and implications for the Lorentz structure of the weak interaction

The KARMEN experiment uses the reaction C-12(v(e), e(-))N-12(g.s.) to measure the energy distribution of v(e) emitted in muon decay at rest mu(+) --> e(+) + v(e) + (v) over bar(mu). The v(e) analog omega(L) of the famous Michel parameter rho has been derived from a maximum-likelihood analysis of events near the kinematic end point, E-max = 52.8 MeV. The result, omega(L) = (2.7(-3.3)(+3.8) +/- 3.1) x 10(-2), is in good agreement with the standard model prediction omega(L) = 0. We deduce a 90% confidence upper limit of omega(L) less than or equal to 0.113, which corresponds to a limit of \g(RL)(S) + 2g(RL)(T)\ less than or equal to 0.78 on the interference term between scalar and tensor coupling constants.