Lepton-number violation and right-handed neutrinos in Higgs-less effective theories

Following previous work, we identify a symmetry Snat that generalizes the concept of custodial symmetry, keeping under control deviations from the Standard Model (SM). To realize Snat linearly, the space of gauge fields has to be extended. Covariant constraints formulated in terms of spurions reduce Snat back to SU (2)L × U(1)Y . This allows for a covariant introduction of explicit Snat-breaking parameters. We assume that Snat is at play in a theory of electroweak symmetry-breaking without a light Higgs particle. We describe some consequences of this assumption, using a non-decoupling effective theory in which the loop expansion procedure is based on both momentum and spurion power counting, as in Chiral Perturbation Theory. A hierarchy of lepton-number violating effects follows. Leading corrections to the SM are non-oblique. The effective theory includes stable light right-handed neutrinos, with an unbroken Z2 symmetry forbidding neutrino Dirac masses. νR contribution to dark matter places bounds on their masses. ∗Unité mixte de recherche 8608 du CNRS.