Constraints on Heavy Neutral Leptons interacting with a singlet scalar

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 → sν 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 → + − decays in the detectors. In addition, we update and correct some errors in the analysis of the original constraints, in the absence of the singlet.