Improved Search for Heavy Neutrinos in the Decay $\pi\rightarrow e\nu$

TL;DR

This paper reports a sensitive search for heavy neutrinos in pion decay, setting new upper limits on neutrino mixing parameters in the 60-135 MeV/c^2 mass range, with no evidence of their existence.

Contribution

It provides the first high-sensitivity search for heavy neutrinos in pion decay within this mass range, improving existing limits significantly.

Findings

No evidence of heavy neutrinos was observed.

Upper limits on the mixing matrix element |U_{ei}|^2 were improved by an order of magnitude.

Constraints on heavy neutrino properties in the 60-135 MeV/c^2 mass range were established.

Abstract

A search for massive neutrinos has been made in the decay $\pi\rightarrow e^+ \nu$. No evidence was found for extra peaks in the positron energy spectrum indicative of pion decays involving massive neutrinos ($\pi\rightarrow e^+ \nu_h$). Upper limits (90 \% C.L.) on the neutrino mixing matrix element $|U_{ei}|^2$ in the neutrino mass region 60--135 MeV/$c^2$ were set, which are %representing an order of magnitude improvement over previous results.