Paper detail

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

The KARMEN experiment uses the reaction 12C(ν_e,e-)12N to measure the energy distribution of ν_e emitted in muon decay at rest. The ν_e analog ω_l of the famous Michel parameter ρhas been derived from a maximum-likelihood analysis of events near the kinematic end point, E_max. The result, ω_l = (2.7^{+3.8}_{-3.3}\pm 3.1) * 10^{-2}, is in good agreement with the standard model prediction ω_l= 0. We deduce a 90% confidence upper limit of ω_l <= 0.113, which corresponds to a limit of |g^S_RL + 2 g^T_RL| <= 0.78 on the interference term between scalar and tensor coupling constants.