SUSY QCD And CP Violation Effects In Top Pair Production Associated With Z~0/h~0 At LHC

In spite of the tremendous achievements of the Standard Model(SM) in under-standingobservedexperiments, it isstill unsatisfieddue tomany fundamentalquestionsunanswered. Among various extensions of the SM, the Minimal Supersymmetric Stan-dard Model(MSSM) is considered to be the most promising one, as it can triumphantlysolve the fine tuning and gauge coupling unification problems in the SM. Furthermore,in the R parity conserving MSSM, the lightest supersymmetric particle(LSP) will benaturally served as the dark matter candidate. Since there is still no clear evidence forthe existence of supersymmetry(SUSY) particles, it is tempting to study the direct andindirect effects of SUSY particle at the large hadron collider(LHC).In this thesis, we precisely studied the supersymmetric QCD(SUSY QCD) effectsin the top pair(t(t|ˉ)) production associated with Z⁰ at the LHC. As the heaviest fermionin the SM, top quark is speculated to play an important role in understanding the elec-troweak symmetry breaking mechanism and searching for new physics. Fortunately,there will be large amount of top quark events produced at the LHC. In MSSM, theSUSY particles can have effects on the t(t|ˉ)Z⁰ production through the loop correction.Therefore, it provides a good opportunity to search for the SUSY signal and test thegauge coupling between top quark and Z⁰ at the LHC. Our results show that the de-pendence of tree-level cross section on the scale(μ=μR =μF) can be significantlyreduced by the complete SUSY QCD correction. The negative contribution from theSUSY particles makes the total cross section be smaller than the NLO QCD correctionof SM. Meanwhile, the transverse momentum and invariant mass distributions of finalstates are distorted due to the resonant effect from gluino pair in loop diagrams. For90GeV< (p_T^t) <120GeV or 120GeV< (p_T^Z0)<150GeV, the negative contribution fromSUSY particles will exceed 8%, which may be observed at the LHC. In the SM, the CPviolation effect induced by the CKM matrix is very small. However, the new CP viola-tion that arises from the complex SUSY parameters in MSSM will make the observableeffect of CP violation in top quark at the LHC. Therefore, any observation of CP viola-tion effects related to the top quark may be served as the evidence of the new physics.Inviewoftheabove, WealsostudiedtheCPviolationintheprocesspp→t(t|ˉ)Z⁰ incom-plexMSSM(cMSSM).BecauseoftheidenticalcollidingparticlesinincomingbeamsattheLHC,inthisworkweconstructaCP-oddobservableA? todiscribetheCP-violatingeffect. Under the parameter space allowed by the current experiments, we investigated the CP violating effect in the process pp→t(t|ˉ)Z⁰, and found that the value of A_Φcanoptimally reach 2.17×10^-3, which may be detected at the LHC.Due to the large Yukawa coupling between top quark and Higgs boson, the pro-duction of t(t|ˉ)h⁰ is an important process in probing the Higgs mechanism and searchingfor the Higgs boson at the LHC. In the thesis, we also studied the contribution from theSUSY particles in one loop diagrams to the process pp→t(t|ˉ)h⁰ in detail, and further dis-cussed the CP violation in this process. The main contents include the SUSY QCD cor-rection to the total cross section and the transverse momentum distributions of the finalparticles in the process pp→t(t|ˉ)h⁰ in the MSSMs with and without complex parametersrespectively. We discussed its CP-violating effects in the complex MSSM(cMSSM).For the CP conserving and violating cases, our results show that the contribution fromthe SUSY QCD particles can reach ?14.86% and ?9.18% respectively. For both mod-els, the dependence of total cross section on the scale(μ) can be significantly reducedby the complete SUSY QCD corrections. We also investigate the distribution of theangle between the final top-quark and the anti-top quark, and find that most of the finalanti-top-quarks favor to go out along the same and opposite directions of outgoing top-quark, particularly the former direction. Finally, we studied the CP violating effects inthe process pp→t(t|ˉ)h⁰ by means of the observable A_Φ, and found the optimal value ofA_Φcan reach 2.83×10^-3 under the current allowed parameters. All above SUSY-QCDeffects could be detected at the LHC.This thesis is organized as follows: in chapter one, we briefly introduce the frame-work of the SM and its problems; in chapter two, a description of the MSSM and CPviolating mechanism in cMSSM are given; in chapter three and four, we investigate theoneloopSUSYQCDcorrectionandCPviolationeffectsintheprocessespp→t(t|ˉ)Z⁰/h⁰at the LHC, respectively; Finally, a short summary is presented.The main innovations in this thesis are listed below:·We carry out a precise study of the NLO SUSY QCD corrections to pp→t(t|ˉ)Z⁰and pp→t(t|ˉ)h⁰ processes at the LHC in both the MSSM and cMSSM for thefirst time. We investigate not only the SUSY QCD corrections with real SUSYparameters, but also introduce the complex SUSY parameters to study their CP-violating effect on the process. Meanwhile, we also apply the complex SUSYparameters to the process pp→t(t|ˉ)h⁰ for the first time. Our calculations anddiscussions provide significant information on the SUSY signatures and the CP-violating effects in the t(t|ˉ)Z⁰/h⁰ production processes at the LHC. ·Because of the identical colliding particles in incoming beams at the LHC, weconstruct a CP-odd observable A_Φto describe the CP-violating effect. This ob-servable is suitable for describing the CP-violating effect in the process of pp→f (f|ˉ)X⁰ at the LHC. We calculate the CP violation effect in the processes pp→t(t|ˉ)Z⁰/h⁰ in the cMSSM. Results show that the CP violation effect in both pro-cesses could be observed at the LHC.