Research On Relevant Issues Of Scalar Dark Matter Phenomenology,Radiative Neutrino Masses And Cosmic Baryon Asymmetry

Extensive astronomical and cosmological observations have offered overwhelming evi-dence for the existence of nonluminous dark matter.In order to address its unknown com-position and provenance from the perspective of particle physics,a profusion of well-motivated new physics have been proposed beyond the SU（3）_c×SU（2）_L×U（1）_Ystandard model in which the particle nature also diversifies.However,if we are specially focused on model building for dark matter itself and take as criteria the minimality of the model,in terms of new fields and parameters,a systematic study of all possibilities becomes feasible and can be categorized into the framework of Minimal Dark Matter.This model extends the standard model by adding an extra SU（2）_Lsinglet or multiplet,containing a neutral dark matter candidate field.The only gauge interactions render the framework particularly predictive,the only free parameter is the dark matter mass constrained by the observed relic density.Considering only the gauge induced processes in such a way is fully justified for an extra fermion multiplet because no other renor-malizable interactions with the standard model particles can be written.However,for a scalar multiplet this assumption is not at all automatic as quartic Higgs portal interactions are perfectly allowed.In this dissertation we have performed a systematic study on the inert Higgs triplet dark matter scenario with special emphasis on investigating some phenomenological effects from the Higgs portal interactions.Our computation shows the interference between the Higgs portal and gauge interactions can result in a drastic decrease of the dark matter-nucleon scattering cross section,thus nullifying experimental constraints from direct detection.The neutral scalar from a multiplet with non-zero hypercharge couples to the Z⁰boson so that it would have a much enhanced spin independent cross section above the present bounds.To evade this constraint,a viable multiplet dark matter entails either zero hypercharge,for instance the real triplet scenario,or a mass splitting between the real and imaginary parts to efficiently suppress the inelastic scattering off nuclei.For the first time,we propose a mixed inert scalar triplet dark matter scenario where a complex scalar triplet with non-zero hypercharge and a real scalar triplet with zero hypercharge mix with each other through their renormalizable coupling to the standard model Higgs doublet.This mixing mechanism provides crucial mass splitting,thus reviving the role of dark matter arising from a complex triplet.For demonstration we consider three limiting cases,i.e.the dominant real triplet scenario,the dominant complex triplet scenario and the democratic real and complex triplet scenario.We conduct analytical computations in these cases with respect to relic density and elastic dark matter-nucleon scattering cross section.And for cross checks,we perform a numerical analysis of relic density in the full parameter space.The results are in well agreement with the analytical computations and evidently show that the three specified cases actually carry most of the relevant features of the full parameter space.On the other hand,the atmospheric,solar,accelerator and reactor neutrino experiments have established the facts that neutrinos have tiny non-zero masses.The particles for the dark matter may also play an essential role in the generation of the neutrino masses and even the origin of the baryon asymmetry.In the first part of this dissertation,we will consider a class of models with Majorana fermion singlets/triplets,real Higgs singlets/triplets and leptonic Higgs doublets.Our models respect a softly broken lepton number and an exactly conserved Z₂dis-crete symmetry,so that they can only give the Majorana neutrino masses at one-loop level.The interactions for the neutrino mass generation can also allow the asymmetric decays of the heavier new fields to realize a successful leptogenesis for the cosmic baryon asymmetry.In the second part of the mixed inert scalar triplet model,we further introduce two types of fermion doublets with opposite hypercharges to construct some heavy Dirac fermions.Thanks to the Yukawa couplings of the inert scalar triplets and fermion doublets to the standard model lepton doublets,we can realize radiative neutrino masses and furthermore,the leptogenesis here pos-sesses the new feature that the lepton asymmetry is induced by the decays of some heavy Dirac fermions rather than the usual Majorana ones.