Search for heavy neutrinos in $K^+\to\mu^+\nu_H$ decays

TL;DR

This study searched for evidence of heavy neutrinos in kaon decays using the E949 data, setting upper limits on their mixing with muon neutrinos in the 175-300 MeV/c^2 mass range.

Contribution

First search for heavy neutrinos in $K^+ o\mu^+ u_H$ decays using E949 data, establishing new upper limits on mixing parameters.

Findings

Upper limits on $|U_{\mu H}|^2$ at $10^{-7}$ to $10^{-9}$

No evidence of heavy neutrinos found in the analyzed mass range

Background from radiative decay was effectively suppressed

Abstract

Evidence of a heavy neutrino, $\nu_H$, in the $K^+\to\mu^+\nu_H$ decays was sought using the E949 experimental data with an exposure of $1.70\times 10^{12}$ stopped kaons. With the major background from the radiative $K^+\to\mu^+\nu_\mu\gamma$ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between muon and heavy neutrino, $|U_{\mu H}|^2$, were set at the level of $10^{-7}$ to $10^{-9}$ for the heavy neutrino mass region 175 to 300 MeV/$c^2$.