Projected NA62 sensitivity to heavy neutral lepton production in $K^+ \to \pi^0 e^+ N$ decays

TL;DR

This paper explores how the NA62 experiment can enhance constraints on heavy neutral leptons by analyzing specific kaon decays, potentially improving existing bounds significantly with current and future data.

Contribution

It demonstrates the potential of NA62 to set new limits on heavy neutral leptons in a previously less explored mass range through missing mass searches.

Findings

Current data can reach a limit of |U_e|^2 ≈ 2×10^{-6} in 125–144 MeV range.

Future data and improvements could improve the limit by an order of magnitude.

NA62 can probe new parameter space for heavy neutral leptons.

Abstract

Heavy neutral leptons (HNLs) appear in many extensions of the Standard Model of particle physics. In this study, we investigate to which extent the NA62 experiment at CERN could improve the existing bounds on the HNL mixing angle $|U_e|^2$ by performing a missing mass search in $K^+ \to \pi^0 e^+ N$ decays in flight. We show that the limit $|U_e|^2 \simeq 2\times 10^{-6}$ can be reached with the currently available data in the mass range 125 -- 144 MeV, which is currently not well covered by production searches. Future data, together with a dedicated trigger and/or improvements in rejection of out-of-acceptance photons, can improve this limit by another order of magnitude.