Low Scale Neutrino Mass Generation and Phenomenological Implications
Vedran Brdar (MPI Heidelberg)

In the first part of the talk I will discuss the low scale (10 - 100 TeV) left-right symmetric model with “naturally” small neutrino masses generated through the inverse seesaw mechanism. The inverse seesaw implies the existence of novel fermion singlets, S, with Majorana mass terms as well as the "left" and "right" Higgs doublets. These doublets provide the portal for S and break the left-right symmetry. The generic feature of the model is the appearance of heavy pseudo-Dirac fermions, formed by S and the right-handed neutrinos, which have the masses in the 1 GeV - 100 TeV range and can be searched at both current and future experiments such as LHC, SHiP, DUNE, and FCC-ee. In the second part of the talk I will introduce the "neutrino option", a recent proposal that the electroweak hierarchy problem is absent if the generation of the Higgs potential stems exclusively from quantum effects of heavy right-handed neutrinos which can also generate active neutrino masses via the type-I seesaw mechanism. In this framework, the tree-level scalar potential is assumed to vanish at high energies. Such a scenario therefore lends itself particularly well to be embedded in a classically scale-invariant theory. I will demonstrate that the minimal scale-invariant framework compatible with the "neutrino option" requires the Standard Model to be extended by two real scalar singlet fields in addition to right-handed neutrinos. In addition, the phase transition connected with scale symmetry breaking is of strong first order with a substantial amount of supercooling. This yields a sizable gravitational wave signal, so that the model can be fully tested by present and future gravitational wave observatories.