The Phenomenological Aspects Of Dark Matter And Minimal Cascade Seesaw Model

The existence of particle dark matter (DM) and nonzero neutrino mass are strong evidence for the new physics. In this thesis, we explore some phenomenological aspects of these topics.The first part will focus on phenomenological study on DM. It is well known bout80%of the matter content of the universe is DM. However, the particle origin of DM is yet to be established. Many extensions of the Standard Model (SM) contain candidates of DM. After systematically review the theoretical backgrounds and different search strategies for this particle, we consider the possibility that a spin-3/2particle acts as DM. Employing the approach of effective field theory, we list all possible4-fermion ef-fective operators between a pair of such fields and a pair of ordinary fermion fields. We investigate the implications of the proposal on the relic density, the antiproton to proton flux ratio in cosmic rays, and the elastic scattering off nuclei in direct detection. Using the observed data and experimental bounds, we set constraints on the relation of cou-plings and dark particle mass. In particular, we find that some mass ranges can already be excluded by jointly applying the observed relic density on the one side and the mea-sured antiproton to proton flux ratio or the upper bounds from direct detection on the other. Following that, We further consider the spin-3/2DM as a standard model (SM) singlet. This demand reduces the number of independent4-fermion effective operators to four. We then make a comprehensive analysis on possible phenomenological effects for such operators. Besides the experiments which have been considered in previous work, we additionally include collider searches for monojet plus missing transverse en-ergy events and Fermi-LAT gamma rays observation. We found that the current data already set strong constraints on the effective interactions in a complementary manner. The constraint from collider searches is most effective at a relatively low mass of DM, and the antiproton-to-proton flux ratio offers the best bound for a heavy DM, while the spin-independent direct detection is the best in between. For DM mass of order10GeV to l TeV, the effective interaction scale is constrained to be typically above a few tens TeV.The second part is devoted to collider signatures of minimal cascade seesaw model. In this model, the seesaw induces at tree level a neutrino mass operator at di-mension nine, by introducing a quadruple scalar Φ of hypercharge unity and a quintuple fermion E of hypercharge zero. We work in a framework that handles the complicated Yukawa couplings in a nice way without losing generality. All mixing matrices are essentially expressed in terms of the vacuum expectation value of the quadruple scalar vΦ, a free complex parameter t, and known neutrino parameters. We show that the low-energy lepton flavor violating transitions of the charged leptons set strong constraints on the free parameters. The constraints have a significant impact on collider physics, and are incorporated in our signal analysis at the LHC. We investigate the signatures of new particles by surveying potentially important channels. We find that the4j2l±signal is most important for the detection of the scalars and the2l±2l±2j,3l±l±2j and3l±2l±=E/T signals are quite promising for the fermions.