Study Of Quasi-dengenrate Majorana Neutrinos In The Decay Of Kaon And Tauon

In this thesis,we extend the Standard Model of Particle Physics by two sterile neutrinos,and we assume that their masses are of the order of MeV-GeV.They are singlets of SU(2)L(?)U(1)Y.However,they can still participate in the electroweak interactions through the mixing with their light counterparts.This will lead to some processes beyond the Stand Model.We assume that the masses of these two sterile neutrinos are quasidegenerate,then we explore the phenomenological consequences of this assumption.Firstly,we take Kaon as an example,and study the lepton-number-violating(LNV)processes in the rare decays of K,D and B mesons.The decay widths of these sterile neutrinos in certain mass range are calculated.We define all the parameters related to the sterile neutrinos,then we use the assumption that their masses are quasi-degenerate,and finally we succeed in separating the part which contains these parameters from the calculation of the partial decay width of Kaon.Secondly,we derive the expression of the active-sterile mixing parameters and we give the exclusion curves under current experimental results.Meanwhile,we also present the above results when only one sterile neutrino is introduced,and we make a comparsion between these two situations.The difference between these two situations can be characterized by a function,which only depends on the parameters of the second sterile neutrino.Finally,we discuss the CP violation effects caused by the phases in the mixing paramters of the sterile neutrinos.Taking Kaon as an example,we obtain the general expression of CP violation.Analyzing these expressions,we obtain the maximum value of these CP asymmetries.We also find out that generally the CP asymmetry will depend on the mass of sterile neutrinos.For the CP violation effects in the rare decays of Tauon,we discuss two processes which are lepton-number-violating and lepton-flavorviolating(LFV)processes respectively.We consider the CP violation in case that the sterile neutrinos are of Majorana and Dirac nature repectively,and their maximum values are obtained.It turns out that the maximum value of the CP violation is the same with that of the Kaon case.Our conclusion is that the CP violation in such processes can be used as an auxillary tool to identify the nature of intermediate neutrinos,it can also constrain the parameters of sterile neutrinos further.