Improved Limits onν¯eEmission fromμ+Decay

We investigated ${\mu }^{+}$ decays at rest produced at the ISIS beam stop target. Lepton flavor (LF) conservation has been tested by searching for ${\overline{\nu }}_e$ via the detection reaction $p({\overline{\nu }}_e,e^{+})n$. No ${\overline{\nu }}_e$ signal from LF violating ${\mu }^{+}$ decays was identified. We extract upper limits of the branching ratio (BR) for the LF violating decay ${\mu }^{+}\to e^{+}+{\overline{\nu }}_e+\stackrel{(-)}{\nu }$ compared to the standard model (SM) ${\mu }^{+}\to e^{+}+{\nu }_e+{\overline{\nu }}_{\mu }$ decay: $\mathrm{B}\mathrm{R}<0.9(1.7)\times {10}^{-3}$ (90% C.L.) depending on the spectral distribution of ${\overline{\nu }}_e$ characterized by the Michel parameter $\tilde{\mathrm{\rho }}=0.75(0.0)$. These results improve earlier limits by one order of magnitude and restrict extensions of the SM in which ${\overline{\nu }}_e$ emission from ${\mu }^{+}$ decay is allowed with considerable strength. The decay ${\mu }^{+}\to e^{+}+{\overline{\nu }}_e+{\nu }_{\mu }$ often proposed as a potential source for the ${\overline{\nu }}_e$ signal observed in the LSND experiment can be excluded.