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

Abstract

We propose an extension of the standard model with a B-L global symmetry that is broken softly at the TeV scale. The neutrinos acquire masses through a type-II seesaw while the lepton (L) asymmetry arises in the singlet sector but without B-L-number violation. The model has the virtue that the scale of L-number violation (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 and leptogenesis at the TeV scale. The stability of the dark matter is ensured by a surviving Z₂ symmetry, which could be lifted at the Planck scale, thereby allowing Planck scale suppressed decay of singlet scalar dark matter particles of mass ≈ 3 MeV to e⁺e⁻ pairs in the Galactic halo. The model also predicts a few hundred GeV doubly charged scalar and a long-lived charged fermion, whose decay can be studied at the Large Hadron Collider (LHC) and International Linear Collider (ILC).