# Search for Heavy Neutrinos in $K^+ \rightarrow \mu^+ \nu_{\mu}$ Decays

## TL;DR

The NA62 experiment searched for heavy neutrinos in kaon decays, setting new upper limits on their mixing with muons for masses between 300-375 MeV/c^2, thus extending previous constraints.

## Contribution

This study provides the first limits on heavy neutrino mixing in the 300-375 MeV/c^2 mass range using kaon decay data.

## Key findings

- No signal observed in the missing mass spectrum.
- Limits on |U_{μ4}|^2 range from 2×10^{-6} to 10^{-5}.
- Extended the mass range of existing heavy neutrino constraints.

## Abstract

The NA62 experiment recorded a large sample of $K^+ \rightarrow \mu^+ \nu_{\mu}$ decays in 2007. A peak search has been performed in the reconstructed missing mass spectrum. In the absence of a signal, limits in the range $2 \times 10^{-6}$ to $10^{-5}$ have been set on the squared mixing matrix element $|U_{\mu4} |^2$ between muon and heavy neutrino states, for heavy neutrino masses in the range 300-375 MeV/$c^2$. The result extends the range of masses for which upper limits have been set on the value of $|U_{\mu4} |^2$ in previous production search experiments.