Improved limits on nuebar emission from mu+ decay

TL;DR

This study sets new upper limits on electron antineutrino emission from mu+ decays, significantly improving previous constraints and impacting models allowing such lepton flavor violating processes.

Contribution

It provides the most stringent upper limits to date on ueb emission from mu+ decay, constraining extensions of the Standard Model that predict such violations.

Findings

No ueb signal detected from mu+ decays.

Upper limit on branching ratio BR < 0.9(1.7) x 10^{-3} at 90% CL.

Excludes ueb emission from mu+ decay as source for LSND ueb signal.

Abstract

We investigated mu+ decays at rest produced at the ISIS beam stop target. Lepton flavor (LF) conservation has been tested by searching for \nueb via the detection reaction p(\nueb,e+)n. No \nueb signal from LF violating mu+ decays was identified. We extract upper limits of the branching ratio for the LF violating decay mu+ -> e+ \nueb \nu compared to the Standard Model (SM) mu+ -> e+ nue numub decay: BR < 0.9(1.7)x10^{-3} (90%CL) depending on the spectral distribution of \nueb characterized by the Michel parameter rho=0.75 (0.0). These results improve earlier limits by one order of magnitude and restrict extensions of the SM in which \nueb emission from mu+ decay is allowed with considerable strength. The decay \mupdeb as source for the \nueb signal observed in the LSND experiment can be excluded.