No abstract

We discuss the damping of inflationary gravitational waves (GW) that reenter the horizon before or during an epoch, where the energy budget of the universe is dominated by an unstable right handed neutrino (RHN), whose out of equilibrium decay releases entropy. Starting from the minimal Standard Model extension, motivated by the observed neutrino mass scale, with nothing more than 3 RHN for the Seesaw mechanism, we discuss the conditions for high scale leptogenesis assuming a thermal initial population of RHN. We further address the associated production of potentially light non-thermal dark matter and a potential component of dark radiation from the same RHN decay. One of our main findings is that the frequency, above which the damping of the tensor modes is potentially observable, is completely determined by successful leptogenesis and a Davidson-Ibarra type bound to be at around 0.1 Hz. To quantify the detection prospects of this GW background for various proposed interferometers such as AEDGE, BBO, DECIGO, Einstein Telescope or LISA we compute the signal-to-noise ratio (SNR). This allows us to investigate the viable parameter space of our model, spanned by the mass of the decaying RHN M 1 2.4 × 10 8 GeV • 2 × 10 −7 eV/ m1 (for leptogenesis) and the effective neutrino mass parameterizing its decay width m1 < 2.9 × 10 −7 eV (for RHN matter domination). Thus gravitational wave astronomy is a novel way to probe both the Seesaw and the leptogenesis scale, which are completely inaccessible to laboratory experiments in high scale scenarios.

Inspired by the S.M.A.S.H. framework we construct a model that addresses the strong CP problem, axion dark matter, inflation and Dirac neutrino masses as well as leptogenesis. The model possesses only two dynamical scales, namely the SM breaking scale vH and the Peccei Quinn (PQ) breaking scale v . We introduce heavy vector-like quarks in the usual KSVZ fashion to implement the PQ mechanism for the strong CP problem. To generate neutrino masses via a dimension six operator scaling as mν ∼ v 3 H / v 2 σ we add heavy triplet and doublet leptons, which are vector-like under the SM but chiral under PQ symmetry. The model is free from the cosmological domain wall problem and predicts an axion to photon coupling which is about an order of magnitude larger than in conventional DFSZ and KSVZ models. Thus our scenario can be probed and potentially excluded by current and next generation axion experiments such as ORGAN or MADMAX. In addition we numerically demonstrate that our construction can generate the observed baryon asymmetry by realizing a version of the Dirac-Leptogenesis scenario. As a consequence of our neutrino mass mechanism we find that the asymmetry in triplet fermion decays can also be significantly enhanced by up to six orders of magnitude when compared to typical Seesaw scenarios without needing to invoke a resonant enhancement. In passing we note that a decaying Dirac fermion with multiple decay modes contains all the necessary ingredients required for the “quasi optimal efficiency”-scenario previously encountered in the context decaying scalar triplets. The impact of the right handed neutrinos and the axion on ΔN eff is estimated and lies within current bounds.

We extend the Dirac Scotogenic model with the aim of realizing neutrino masses together with the mass of a keV-scale dark matter (DM) candidate via the same one-loop topology. Two of the Standard Model (SM) neutrinos become massive Dirac fermions while the third one remains massless. Our particle content is motivated by an anomaly free U(1) B-L gauge symmetry with exotic irrational charges and we need to enforce an additional Z 5 symmetry. The dark matter candidate does not mix with the active neutrinos and does not have any decay modes to SM particles. DM is produced together with dark radiation in the form of right handed neutrinos via out of equilibrium annihilations of the SM fermions mediated by the heavy B-L gauge boson. In order to avoid DM over-production from Higgs decays and to comply with Lyman-α bounds we work in a low temperature reheating scenario with 4 MeV T RH 5 GeV. Our setup predicts a contribution to ∆N eff. that decreases for larger DM masses and is below the sensitivity of upcoming precision measurements such as CMB-S4. A future observation of a signal with ∆N eff.0.012 would exclude our scenario. We further sketch how inflation, reheating and Affleck-Dine baryogenesis can also be potentially realized in this unified framework.