Theoretical Studies For Beyond The Standard Model Phenomenology

The Standard Model of Particle Physics, which contains electroweak theoryand quantum chromodynamics (QCD), is the theory of fundamental interactionsbetween elementary particles. During the past forty years, it has been extensivelytested and confirmed with amazing accuracy. The discovery of neutral weak in-teractions and the production of intermediate vector bosons (W±and Z) withthe expected properties increased our confidence in the model. However, it pos-sesses some problems whose solution could imply physics beyond its scope. Thestudy of issues beyond the standard model should not be viewed as a marginalactivity in high energy physics. In this thesis we will consider the following threeaspects, anomalously asymmetry Bs Bˉssystem in B physics, dark matter andneutrino physics.The B physics is a good test for the SM and a promising way to explore newphysics beyond the SM. Recently the D0and CDF Collaborations have reportedevidence for an anomalously large dimuon asymmetry Bs Bˉs. In the secondchapter, after analyzing the experimental result, we come to the conclusion thatnew physics beyond the SM is needed. Exchange of unparticle stuf of dimen-sion dUwith FCNC interaction can induce M12,uand Γ12,ucausing meson andanti-meson mixing with the relation Γ12,u/M12,u=2tan(πdU). We show thatthis type of unparticle contribution can provide the much needed large Γ12stoexplain the experimental results. The same interaction can also accommodatelarge mixing induced CP violation in Bs→J/ψφ indicated by CDF and D0data. Experimental data can provide constraints on the unparticle dimensionand scale.In the third chapter, we will study the SM plus a real gauge-singlet scalarfield dubbed darkon (SM+D) which is the simplest model possessing a weakly interacting massive particle (WIMP) dark-matter candidate. In this model, theparameters are constrained from dark matter relic density and direct searches.The fact that interaction between darkon and SM particles is only mediated byHiggs boson exchange may lead to significant modifications to the Higgs bosonproperties. If the dark matter mass is smaller than a half of the Higgs boson mass,the Higgs boson can decay into a pair of darkons resulting in a large invisiblebranching ratio. The Higgs boson will be searched for at the LHC and may wellbe discovered in the near future. If a Higgs boson with a small invisible decaywidth is found, the SM+D model with small dark matter mass will be in trouble.We find that by extending the SM+D to a two-Higgs-doublet model plus a darkon(THDM+D) it is possible to have a Higgs boson with a small invisible branchingratio and at the same time the dark matter can have a low mass by tuning extraparameters. Future DM search experiments can further constrain the parameterspace of the model.In the last part, in Type III seesaw the heavy neutrinos are contained inleptonic triplet representations for the neutrino physics. The Yukawa couplingsof the triplet fermion and the left-handed neutrinos with the doublet Higgs fieldproduces the Dirac mass terms. Besides, we point out that it is also possible tohave a quadruplet Higgs to produce the Dirac mass terms to facilitate the seesawmechanism. The light neutrinos obtain non-zero seesaw masses with tree level.The vacuum expectation value of the quadruplet Higgs is constrained to be verysmall from electroweak precision data. Therefore, if both Higgs contribute to thesame order of the neutrino mass, the Yukawa couplings of a quadruplet can bemuch larger than those for a doublet. We also find that unlike the usual TypeIII seesaw model, with both doublet and quadruplet Higgs representations, justone leptonic triplet is possible to have a phenomenologically acceptable modelbecause light neutrino masses can receive sizable contributions at both tree andone loop levels. Because one-loop contribution can raise the rank of the massmatrix to two. Besides, large Yukawa couplings of the quadruplet can induceobservable efects for lepton flavor violating processes μ→eγ and μ e conver-sion. Implications of the recent μ→eγ limit from MEG and also limit on μ econversion on Au will be discussed. Some interesting collider signatures for thedoubly charged Higgs boson in the quadruplet will also be discussed.