Neutrino Mass Seesaw at the Weak Scale, the Baryon Asymmetry, and the LHC

We consider theories where the Standard Model (SM) neutrinos acquire masses through the seesaw mechanism at the weak scale. We show that in such a scenario, the requirement that any pre-existing baryon asymmetry, regardless of its origin, not be washed out leads to correlations between the pattern of SM neutrino masses and the spectrum of new particles at the weak scale, leading to definite predictions for the LHC. For type I seesaw models with a TeV scale Z coupled to SM neutrinos, we find that for a normal neutrino mass hierarchy, at least one of the right-handed neutrinos must be `electrophobic, decaying with a strong preference into final states with muons and tauons rather than electrons. For inverted or quasi-degenerate mass patterns, on the other hand, we find upper bounds on the mass of at least one right-handed neutrino. In particular, for an inverted mass hierarchy, this bound is 1 TeV, while the corresponding upper limit in the quasi-degenerate case is 300 GeV. Similar results hold in type III seesaw models, albeit with somewhat more stringent bounds. For the Type II seesaw case with a weak scale SU(2) triplet Higgs, we again find that an interesting range of Higgs triplet masses is disallowed by these considerations.

Probing Resonant Leptogenesis at the LHC

We explore direct collider probes of the resonant leptogenesis mechanism for the origin of matter. We work in the context of theories where the Standard Model is extended to include an additional gauged U(1) symmetry broken at the TeV scale, and where the light neutrinos obtain mass through a Type I seesaw at this scale. The CP asymmetry that generates the observed matter-antimatter asymmetry manifests itself in a difference between the number of positive and negative like-sign dileptons N(ell^+ell^+)-N(ell^-ell^-) that arise in the decay of the new Z gauge boson to two right-handed neutrinos N, and their subsequent decay to leptons. The relatively low efficiency of resonant leptogenesis in this class of models implies that the CP asymmetry, epsilon, is required to be sizable, i.e. of order one. In particular, from the sign of the baryon asymmetry of the Universe, emph{an excess of antileptons is predicted}. We identify the domains in M_{Z}--M_N space where such a direct test is possible and find that with 300~fb^{-1} of data and no excess found, the LHC can set the $2sigma$ exclusion limit epsilon lesssim 0.22.