Search for heavy neutral leptons in $\pi^+$ decays to positrons

TL;DR

This paper reports a search for heavy neutral leptons produced in pion decays, setting new upper limits on their mixing parameters in the 95-126 MeV/c^2 mass range, based on data from the NA62 experiment.

Contribution

The study provides the first constraints on heavy neutral leptons in pion decays within this mass range using in-flight decay data from NA62.

Findings

Upper limits on $|U_{e4}|^2$ at $10^{-8}$ level

Mass range sensitivity from 95 to 126 MeV/c^2

Lifetime exceeding 50 ns constraints established

Abstract

A search for heavy neutral lepton ($N$) production in $\pi^+\to e^+ N$ in-flight decays using data collected by the NA62 experiment at CERN in 2017-2024 is reported. Upper limits for the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{-8}$ for heavy neutral leptons with mass in the range 95-126 $MeV/c^2$ and lifetime exceeding 50 ns.