The Quest for Heavy Neutral Leptons

As mentioned earlier in Sect.  2.2 , the existence of new neutral leptons is a key aspect in various extensions of the Standard Model, aimed at accommodating the observed neutrino data. For instance, at least two \(\nu _R\) are required to accommodate light neutrino masses via the type-I seesaw mechanism [1–5]. Moreover, in several variants of the type-I seesaw realized at low scale, other sterile fermions \(\nu _S\) are considered, as in the case for the Inverse [6–8] and Linear [9, 10] seesaw mechanisms; these variants allow having large neutrino Yukawa couplings with a seesaw scale potentially within collider reach.