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

TL;DR

The paper measures the energy spectrum of electron neutrinos from muon decay to test the Lorentz structure of the weak interaction, finding results consistent with the Standard Model and setting limits on scalar and tensor couplings.

Contribution

It provides a novel measurement of the neutrino energy spectrum from muon decay and constrains possible non-standard Lorentz-violating interactions.

Findings

Measured u_e energy spectrum using the KARMEN experiment.

Derived the u_e analog of the Michel parameter, _l, consistent with the Standard Model.

Set upper limits on scalar and tensor coupling interference terms.

Abstract

The KARMEN experiment uses the reaction 12C(\nu_e,e-)12N to measure the energy distribution of \nu_e emitted in muon decay at rest. The \nu_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. The result, \omega_l = (2.7^{+3.8}_{-3.3}\pm 3.1) * 10^{-2}, is in good agreement with the standard model prediction \omega_l= 0. We deduce a 90% confidence upper limit of \omega_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.