Measurement of the energy spectrum of e from muon decay and implications for the Lorentz structure of the weak interaction
Abstract
The KARMEN experiment uses the reaction 12C(e,e-)12N to measure the energy distribution of e emitted in muon decay at rest. The e analog ωl of the famous Michel parameter has been derived from a maximum-likelihood analysis of events near the kinematic end point, Emax. The result, ωl = (2.7+3.8-3.3 3.1) * 10-2, is in good agreement with the standard model prediction ωl= 0. We deduce a 90% confidence upper limit of ωl <= 0.113, which corresponds to a limit of |gSRL + 2 gTRL| <= 0.78 on the interference term between scalar and tensor coupling constants.
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