WIMPy Leptogenesis in Non-Standard Cosmologies

We study the possibility of generating baryon asymmetry of the universe from dark matter (DM) annihilations during non-standard cosmological epochs. Considering the DM to be of weakly interacting massive particle (WIMP) type, the generation of baryon asymmetry via leptogenesis route is studied where WIMP DM annihilation produces a non-zero lepton asymmetry. Adopting a minimal particle physics model to realise this along with non-zero light neutrino masses, we consider three different types of non-standard cosmic history namely, (i) fast expanding universe, (ii) early matter domination and (iii) scalar-tensor theory of gravity. By solving the appropriate Boltzmann equations incorporating such non-standard history, we find that the allowed parameter space consistent with DM relic and observed baryon asymmetry gets enlarged with the possibility of lower DM mass in some scenarios. While such lighter DM can face further scrutiny at direct search experiments, the non-standard epochs offer complementary probes on their own.Comment: 50 pages, 21 captioned figures, matches version accepted for publication in JCA

Low Scale Leptogenesis in Singlet-Triplet Scotogenic Model

The scotogenic model presents an elegant and succinct framework for elucidating the origin of tiny neutrino masses within the framework of the Standard Model, employing radiative corrections within the domain of the dark sector. We investigate the possibility of achieving low-scale leptogenesis in the singlet-triplet scotogenic model (STSM), where dark matter mediates neutrino mass generation. We initially considered a scenario involving two moderately hierarchical heavy fermions, N and $\Sigma$, wherein the lepton asymmetry is generated by the out-of-equilibrium decay of both particles. Our analysis indicates that the scale of leptogenesis in this scenario is similar to that of standard thermal leptogenesis and is approximately $M_{N,\Sigma}\sim 10^{9}$ GeV, which is comparable to the Type-I seesaw case. Further, we consider the case with three heavy fermions ($N_1$, $N_2$, and $\Sigma$) with the hierarchy $M_{N_{1}} < M_{\Sigma} \ll M_{N_{2}}$, which yields the lower bound on heavy fermions up to 3.1 TeV, therefore significantly reduce the scale of the leptogenesis up to TeV scale. The only prerequisite is suppression in the $N_{1}$ and $\Sigma$ Yukawa couplings, which causes suppressed washout effects and a small active neutrino mass of about $10^{-5}$ eV. This brings about the fascinating insight that experiments aiming to measure the absolute neutrino mass scale can test low-scale leptogenesis in the scotogenic model. Further, the hyperchargeless scalar triplet $\Omega$ provides an additional contribution to mass of the $W$-boson explaining CDF-II result.Comment: 28 pages, 11 figure

Low scale Dirac leptogenesis and dark matter with observable ΔNeff\Delta N_{\rm eff}

We propose a gauged $B-L$ extension of the standard model (SM) where light neutrinos are of Dirac type by virtue of tiny Yukawa couplings with the SM Higgs. To achieve leptogenesis, we include additional heavy Majorana fermions without introducing any $B-L$ violation by two units. An additional scalar doublet with appropriate $B-L$ charge can allow heavy fermion coupling with the SM leptons so that out of equilibrium decay of the former can lead to generation of lepton asymmetry. Due to the $B-L$ gauge interactions of the decaying fermion, the criteria of successful Dirac leptogenesis can also constrain the gauge sector couplings so as to keep the corresponding washout processes under control. The same $B-L$ gauge sector parameter space can also be constrained from dark matter requirements if the latter is assumed to be a SM singlet particle with non-zero $B-L$ charge. The same $B-L$ gauge interactions also lead to additional thermalised relativistic degrees of freedom $\Delta N_{\rm eff}$ from light Dirac neutrinos which are tightly constrained by Planck 2018 data. While there exists parameter space from the criteria of successful low scale Dirac leptogenesis, dark matter and $\Delta N_{\rm eff}$ even after incorporating the latest collider bounds, all the currently allowed parameters can be probed by future measurements of $\Delta N_{\rm eff}$.Comment: 32 pages, 7 figure