# Search for Heavy Neutrinos in $\pi \to \mu\nu$ Decay

## TL;DR

This paper searches for heavy neutrinos in pion decays by analyzing muon energy spectra, finding no evidence but setting new upper limits on neutrino mixing in the 15.7--33.8 MeV/c^2 mass range.

## Contribution

The study provides the first search for heavy neutrinos in pion decay within this mass range and improves existing upper limits by an order of magnitude.

## Key findings

- No evidence for heavy neutrinos was observed.
- Upper limits on neutrino mixing matrix elements were significantly improved.
- The results constrain models predicting heavy neutrinos in this mass range.

## Abstract

Heavy neutrinos were sought in pion decays $\pi^+ \rightarrow \mu^+ \nu$ by examining the observed muon energy spectrum for extra peaks in addition to the expected peak for a massless neutrino. No evidence for heavy neutrinos was observed. Upper limits were set on the neutrino mixing matrix $|U_{\mu i}|^2$ in the neutrino mass region of 15.7--33.8 MeV/c$^2$, improving on previous results by an order of magnitude.