Type-II seesaws at colliders, lepton asymmetry and singlet scalar dark matter

Abstract

We propose an extension of the standard model with aB–L global symmetry that is broken softly at the TeV scale. Theneutrinos acquire masses through a type-II seesaw while the lepton(L) asymmetry arises in the singlet sector but withoutB–L-number violation. The model has the virtue that the scale ofL-numberviolation (Λ) giving rise to neutrino masses is independent of the scale of leptogenesis(Λ′). As a result the model can explain neutrino masses, singlet scalar dark matter andleptogenesis at the TeV scale. The stability of the dark matter is ensured by a survivingZ2 symmetry, which could be lifted at the Planck scale, thereby allowingPlanck scale suppressed decay of singlet scalar dark matter particles of mass≈3 MeVto e+e− pairs in the Galactic halo. The model also predicts a few hundred GeV doublycharged scalar and a long-lived charged fermion, whose decay can be studiedat the Large Hadron Collider (LHC) and International Linear Collider (ILC).