Studies On Matter-Antimatter Asymmetry,Neutrino Mass And Dark Matter

One of the greatest goals of scientists is to explore the fundamental building blocks of matter and how the universe works.The Standard Model is up to now the most successful theory about elementary particles and their interactions including electromagnetic,weak and strong interaction.It contains three kinds of particles,that is fermions building up the material,gauge bosons mediating interactions and Higg boson giving masses to elementary particles.Withstanding the wide-ranging experimental tests,the Standard Model has its own problems,such as neutrino mass problems,dark matter,dark energy and hierarchy problems,which all need theories beyond the Standard Model to explain.This thesis first introduces basic knowledge about Standard Model,neutrino oscillation and neutrino mass problems,especially about the seesaw models.Then baryon asymmetry in the universe is discussed,including observations on this phenomenon and feasible resolutions,one of most popular is leptogenesis.The lepton asymmetry can then be transformed partially to baryon asymmetry through the so-called sphaleron process.We introduce a new model based on leptogenesis,new particles beyond the Standard Model are included:a fermion triplet as the so-called minimal dark matter,three heavy fields including a scalar singlet,a fermion triplet and a fermion singlet/Higgs triplet.The heavy scalar singlet,which is expected to drive an inflation,and the dark matter fermion triplet are odd under an unbroken Z₂discrete symmetry,while the other fields are all even.The heavy fermion triplet offers a tree-level type-III seesaw and then mediates a three-body decay of the inflaton into the standard model leptons,Higgs doublet and the dark matter fermion triplet.The heavy fermion singlet/Higgs triplet not only results in a type-I/II seesaw at tree level but also contributes to the inflaton decay at one-loop level.In this scenario,the type-I/II seesaw contains all of the physical CP phases in the lepton sector and hence the CP violation for the non-thermal leptogenesis by the inflaton decay exactly comes from the imaginary part of the neutrino mass matrix.Thus a unified model for neutrino mass,baryon asymmetry,dark matter and inflation is built.Unlike the traditional leptogenesis which has the same scale as the neutrino mass gener-ation,we show another possibility that a high-scale leptogenesis can be consistent with a low-scale one-loop neutrino mass generation.Our models are based on the SU(3)_c(?)SU(2)_L(?)U(1)_Y(?)U(1)_B-Lgauge groups.Except a complex singlet scalar for the U(1)_B-Lsymme-try breaking,the other new scalars and fermions(one scalar doublet,two or more real scalar singlets/triplets and three right-handed neutrinos)are odd under an unbroken Z₂discrete sym-metry.The real scalar decays can produce an asymmetry stored in the new scalar doublet which subsequently decays into the standard model lepton doublets and the right-handed neutrinos.The lepton asymmetry in the standard model leptons then can be partially converted to a baryon asymmetry by the sphaleron process.By integrating out the heavy scalar singlets/triplets,we can realize an effective theory to generate the small neutrino radiative masses at the Te V scale.On the other hand,leptogenesis can occur at the scale of 10¹⁴Ge V,thus a high-scale leptogenesis is connected with a low-scale neutrino mass generation.Furthermore,the lightest right-handed neutrino can serve as a dark matter candidate.