Precise Tests Of T-t-γ Coupling And Gluino Decay

Although the standard model(SM) is considered as the most successful model of particle physics, the Higgs boson from Higgs Mechanism which is one of the SM corner-stones, has not been discovered yet. Thus probing the origin of electroweak symmetry breaking(EWSB) and searching for new physics are the most important goals in modern particle physics. Fortunately, the Tevatron at Fermilab and the Large Hardon Collider (LHC) at CERN provide us good plats to test the SM precisely and probe new physics. The topic of this thesis follows the trends in both theoretical and experimental particle physics. We study precisely the t-t-γ coupling and gluino Decay.The top-quark is the heaviest elementary fermion in SM whose mass is close to the scale of EWSB. It is natural to assume that the top-quark is most sensitive to EWSB. Hadron colliders can produce copious top events, and the LHC is called a top facto-ry, ttγ hadronic production is sensitive only to t-t-γ coupling and free from t-t-Z coupling at Hadron colliders. Thus precision study on ttγ associated production can support precision test on t-t-γ coupling in theoretical aspect. In chapter3, we present the precision study on associated production of ttγ at hadron colliders including the next-to-leading order(NLO) QCD corrections. We provide the leading order(LO) and NLO cross sections, distributions of the transverse momenta of top-quark and photon. We also study the LO and NLO top-quark charge forward-backward asymmetry(AFB) at the Tevatron. We find that the NLO corrections improve obviously the indepen-dence of the LO cross sections on the unphy sical scale μ. Our numerical results show that the NLO QCD corrections can surpass50%assuming μ=mt at the LHC. Such a remarkable effect must be taken into account for the precise experimental measure-ment at the LHC. Our first published paper was cited by the Tevatron CDF experi-mental group and drew great attentions of the LHC ATLAS experimental group. Ac-cording to the needs of the LHC ATLAS experimental group, we extend the study on ttγ associated production to the7TeV LHC, and probe the impacts from the lower PT,cut(γ) and the exotic top-quark(qt=-4e/3). Our further studies turn out that the K-factor[K≡σNLO/σLO] varies only slightly with the variation of either the PT,cut(γ) or the top-quark electric charge(qt)(about3%for the latter). However the LO and NLO cross sections strongly depend on the pT,cut(γ) and qt.Various new models, such as the Little Higgs Models (LH), the Large Extra Di-mension Model (LED) and the Minimal Supersymmetiy Standard Model (MSSM), are raised to solve the fundamental problems in the SM. Among these new models, the MSSM was regarded as the most promising candidate due to its elegant symmetry and rich physics meaning. According to the MSSM, all the SM particles have their su-persymmetric partners. In the R-conserving MSSM, the lightest neutralino is usually thought to be the lightest supersymmetry particle (LSP). The supersymmetric particles are unstable and decay into lighter particles immediately after productions except L-SP. The supersymmetry partner t1of top quark is regarded as the lightest scalar quark (squark). Thus x10and t1can be probably discovered in experiments. Colored supersym-metry scalar quarks and gluinos can be copiously produced at hardon colliders and de-cay immediately. Depending on the mass spectrum of supersymmetry particles, gluinos may have many decay modes. As one expects that t1is the lightest squark, the decay mode gâ†'ttχ10could have major contribution to the total decay width of gluino. Therefore, the accurate calculations including the NLO corrections to this decay mode are necessary. In chapter4, we present the precision study on the decay of the super-symmetry particle gluino (gâ†'ttχ10) including the NLO QCD corrections. We select the scenario SPS6point as proposed in the SPA project as the supersymmetry parame-ters. In dealing with resonant propagators, we adopt the complex-mass scheme (CMS) which can guarantee gauge invariance. And also we present firstly the corresponding counter-terms in the scalar top sector and their deduction in the CMS. Then, using the SPS6reference point and CMS, we present the LO and NLO QCD width of the decay gâ†'ttχ10. Meanwhile, we study their dependence on the vacuum expectation val-ue(VEVs)(tan β), the mass of gluino(mg), the masses of scalar top quarks (mt1, mt2) and the mass of χ10(mχ1o). We present the LO and NLO width distributions of the tt invariant mass (Mtt) and the missing energy (Emiss). Our numerical results show that, in some parameter space, the absolute relative correction can exceed30%. Such con-siderable effect can provide excellent theoretical foundation for supersymmetry particle hunting.The innovative points in this thesis are mainly listed below:· This is the first systematic study of pp/ppâ†'ttγ+X, including the NLO QCD corrections and the most precise theoretical predictions until now. It provides theoretical foundation for studying ttγ events at the Tevatron and the LHC, espe-cially for the precise measurement of top-photon coupling. Our results have been adopted by Tevatron CDF experiment group at Fermilab. · Our publication "Phys. Rev. D80(2009)014022" has drawn attention in the field of particle physics and has been cited more than10times by others until now. According to the needs of the Atlas experimental group at the LHC, we studied further on this topic and gave the most precise theoretical predictions at the7TeV LHC so far. We also explored the impacts from the exotic top-quark and lower PT,cut(γ) Our results have been adopted by the LHC ATLAS experimental group at CERN.· This is the first precision study on gâ†'ttχ10, including the NLO QCD correc-tions and the most precise theoretical predictions so far. It provides important theoretical foundation in hunting for supersymmetry particles and is essential for further study in the MSSM.· The complex-mass scheme(CMS) is adopted in the precision study on gâ†'ttχ10. It can solve multi-supersymmetry-particle resonance problem. We present firstly the corresponding counter-terms of the scalar top sector and their deduction in the CMS. In dealing with unstable particles, approaches like narrow-width approxi-mation and effective theory violate gauge invariance of theory. On the contrary, the CMS not only solves multi-particle resonance problem, but also guarantees gauge invariance.