Constraints on Heavy Neutral Leptons interacting with a singlet scalar

TL;DR

This paper reinterprets existing experimental constraints on heavy neutral leptons by including their interactions with a singlet scalar, revealing potential for stronger bounds and providing a method to recast constraints efficiently.

Contribution

It introduces a novel approach to reinterpret constraints on HNLs considering singlet scalar interactions, updating previous analyses and correcting errors without extensive simulations.

Findings

Constraints can be weakened or strengthened depending on decay channels.

Observable scalar decays can improve detection prospects.

Method allows recasting of older experimental data efficiently.

Abstract

Heavy neutral leptons (HNLs) are an attractive minimal extension of the Standard Model, as is a singlet scalar $s$ mixing with the Higgs boson. If both are present, it is natural for HNLs to interact with $s$. For a light singlet,the decay $N\to s\nu$ can dominate over weak HNL decays. We reinterpret existing constraints on HNL mixing from the DELPHI, CHARM and Belle experiments for 0.5-100 GeV mass HNLs, taking into account the new decay channel. Although the constraints are typically weakened, in some cases they can become stronger, due to observable $s\to\ell^+\ell^-$ decays in the detectors. The method presented here could be used to recast constraints from other (older) experiments without resorting to computationally expensive Monte Carlo simulations. In addition, we update and correct some errors in the analysis of the original constraints, in the absence of the singlet.