Right-handed neutrinos: seesaw models and signatures

Abstract: We give a pedagogical introduction to right-handed neutrinos as a simple extension to the Standard Model (SM), focussing on seesaw models and their possible experimental signatures. We preface this with a review of the lepton sector of the SM, where charged lepton masses arise from Yukawa couplings and neutrino Majorana masses from the Weinberg operator, leading to a unitary lepton mixing matrix. We first introduce a single right-handed neutrino and the seesaw mechanism, yielding a heavy neutral lepton, then generalise the results to the canonical case of three right-handed neutrinos within a general parameterisation, leading to non-unitary lepton mixing and three heavy neutral leptons, which can detected directly or indirectly via lepton flavour violation or neutrinoless double beta decay. We show how the sequential dominance of three right-handed neutrinos with diagonal masses naturally leads to an effective two right-handed neutrino model with lepton mixing angle predictions in the constrained cases, but unobservable heavy neutral leptons. On the other hand, with degenerate off-diagonal masses, the two right-handed neutrinos can form a single heavy and observable Dirac neutrino, within a two Higgs doublet or Majoron model. Finally we discuss extra singlet neutrinos which can lead to either a double seesaw or an inverse seesaw, depending on their Majorana masses, where the latter allows observable heavy neutral leptons, and the possibility of a minimal inverse seesaw model where mixing angles can be predicted.