Search for the rare decay $K^+\to\mu^+\nu\bar\nu\nu$

TL;DR

This study searched for the rare decay $K^+ o+ uar u u$ using E949 data, setting an upper limit on its decay rate and providing data for various muon spectrum assumptions.

Contribution

First experimental search for the decay $K^+ o+ uar u u$, establishing an upper limit on its decay rate.

Findings

Upper limit on decay rate: $ extless 2.4 imes 10^{-6}$ of total kaon decays.

Data allows calculation of decay rates for different muon spectra.

Analysis dominated by background from $K^+ o+ u_ ext{gamma}$.

Abstract

Evidence of the $K^+\to\mu^+\nu\bar\nu\nu$ decay was searched for using E949 experimental data with an exposure of $1.70\times 10^{12}$ stopped kaons. The data sample is dominated by the backgrond process $K^+\to\mu^+\nu_\mu\gamma$. An upper limit on the decay rate $\Gamma(K^+\to\mu^+\nu\bar\nu\nu)< 2.4\times 10^{-6}\Gamma(K^+\to all)$ at 90% confidence level was set assuming the Standard Model muon spectrum. The data are presented in such a way as to allow calculation of rates for any assumed $\mu^+$ spectrum.