Phenomenological Studies Of New Physics In Hadronic B-meson And Rare Top-quark Decays

An amount of experimental results have proved that the Standard Model(SM)of particle physics is a very successful theory in describing the fundamental particles and their interactions.However,there are still some problems in nature that the SM cannot explain,indicating the presence of new particles and interactions.Direct searches for new particles have been performed by the Large Hadron Collider(LHC)but without any signals yet.On the other hand,we can also investigate the possible new-physics(NP)effects in the precision flavour-physics measurements,or look for the potential NP through flavour-changing neutral-current(FCNC)decays which are sensitive to contributions from NP.In this thesis,we explore the possible NP effects indirectly in the non-leptonic two-body B-meson and rare top-quark decays.Recent measurements of non-leptonic B-meson decays by LHCb and Belle indicate that the deviations between the SM predictions and the experimental measurements of the branching ratios of B0→D(*)+ K-and Bs0→Ds(*)+ π-can reach 4-5σ.Giving such significant deviations,we explore the potential NP effects in these non-leptonic B-meson decays.Firstly,keeping the next-to-next-to-leading-order corrections,we update the branching ratios of the non-leptonic Bmeson decays B(s)0→D(s)(*)+ L-(L∈{π,ρ,K(*)｝)within the framework of QCD factorization.Numerical analysis shows that the deviations between the SM predictions and the experimental measurements of the branching ratios of these decays can reach 4-6σ.In addition,considering the ratios of the non-leptonic B(s)0→D(s)(*)+ L-decay rates with respect to the corresponding differential semi-leptonic B(s)0→D(s)(*)+l-υl decay rates evaluated at q2=mL2 to minimize the impacts of input parameters,such as form factors and the CKM elements,we find that some of the deviations between the theoretical and experimental branching ratios of these decays can even reach 6-8σ.Secondly,in order to study the NP effects model-independently,we perform an analysis of the effects from twenty linearly independent effective four-quark operators that can contribute,either directly or through operator mixing,to b→cūd(s)transitions.We find that,under the combined constraints from the current experimental data,the deviations can be well explained at the 1σ level by the effective operators with V-A(?)V-A structure,and also at the 2σ level by the four-quark operators with S+P(?)S-P and S+P (?)S+P structures,while the NP four-quark operators with other Dirac structures fail to provide a consistent interpretation,even at the 2σ level.On the other hand,as the four-quark operators can be generated by either a colorless charged vector boson(A)or a colorless charged scalar(H),we also perform the analysis of the NP effects in the non-leptonic B-meson decays in a model-dependent way with mA,H=1 TeV.We find that all the deviations observed in the non-leptonic B-meson decays could be explained simultaneously with only λLL(A)for the charged vector boson A,as well as with only λRR(H)or λRL(H)for the charged scalar H,at the 2σ level,while all the other couplings fail in either model.Furthermore,if their left-and right-handed couplings are symmetric or asymmetric,these two models can’t provide a simultaneous explanation the deviations observed in the B(s)0→ D(s)(*)+L-decays,even at the 2σ level.It is well known that the top-quark FCNC decays t→cg(g)are absent at the tree level and suppressed at the loop level by the Glashow-Iliopoulos-Maiani mechanism in the SM.The branching ratios of t→cg and t→cgg are tiny in the SM,which are of the orders of 10-12 and 10-9,respectively,which makes these decays very sensitive to NP contributions.Motivated by these features,we study the top-quark FCNC decays t→cg(g)within the Aligned Two-HiggsDoublet Model(A2HDM).We update the SM predictions of the branching ratios of these two decays,and evaluate their maximum values in the A2HDM as well as in the four conventional Two-Higgs-Doublet Models(2HDMs)with Z2 symmetries.Numerical analysis shows BSM(t→cg)=4.50 × 10-12 and BSM(t→cgg)=8.31×10-10,and we can see that the latter is about two orders of magnitude larger than the former.Taking into account the constraints on the model parameters from a global fit obtained at the 95.5%confidence level,B(t→cg)and B(t→cgg)can reach up to 3.36 ×10-9 and 2.95 × 10-9,respectively,being of the same order in the A2HDM.It can be seen that branching ratio of the two-body decay t→cg in the A2HDM is enhanced by about three orders of magnitude relative to the SM prediction.In addition,these two decays are also investigated in the 2HDMs with Z2 symmetries.We find that the branching ratios of t→cg(g)decays almost remain identical to their SM expectations after considering the flavour constraints on the 2HDMs parameters.In general,the predicted branching ratios of t→cg and t→cgg decays can not be touched by the future colliders(such as high-luminosity LHC)either in A2HDM or in 2HDMs with Z2 symmetries.In short,we study the NP effects in the non-leptonic B-meson decays within the framework of QCD factorization,and find that the deviations between the theoretical predictions and the experimental measurements of the branching ratios of these decays can be explained by the NP operators.Moreover,we also evaluate the branching ratios of rare top-quark decays t→cg(g)in the A2HDM,and find that they can be of the order of 10-9,which still can’t reach the sensitivities of the high-luminosity LHC.In the future,with the running of Belle Ⅱ and LHCb,as well as the progresses of lattice QCD,we look forward to deepening our understanding of the NP effects through the precision flavour physics.