Improved limits on νquerₑ emission from μ⁺ decay

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