Search for heavy neutral lepton production in $K^+$ decays to positrons

TL;DR

This paper reports a search for heavy neutral leptons in $K^+$ decays, setting new upper limits on their mixing parameters and excluding certain mass ranges, thus advancing constraints on neutrino models.

Contribution

The study provides the first stringent upper limits on heavy neutral lepton production in $K^+$ decays within the 144--462 MeV/$c^2$ mass range, surpassing previous bounds.

Findings

Established upper limits on $|U_{e4}|^2$ at $10^{-9}$

Excluded heavy neutral lepton masses up to 340 MeV/$c^2$ based on BBN constraints

Significantly improved previous search sensitivities

Abstract

A search for heavy neutral lepton ($N$) production in $K^+\to e^+N$ decays using the data sample collected by the NA62 experiment at CERN in 2017--2018 is reported. Upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{-9}$ over most of the accessible heavy neutral lepton mass range 144--462 MeV/$c^2$, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The $|U_{e4}|^2$ range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/$c^2$.