Phenomenological Studies Of The B?c(?)s Decays Within The Standard Model

In the last few decades,the Standard Model(SM)of particle physics has become a very successful basic theory.It explains three of the four basic interactions,strong interaction,weak interaction and electromagnetic interaction,except the gravity interaction.So far,the SM has achieved great success in both theory and experiment.In July 2012,the LHC experiment an-nounced the discovery of the last particle predicted by the SM—the Higgs boson.However,there still exist many questions that cannot be explained within the SM,e.g.,the Dark Matter and Dark Energy observed in the Universe.Therefore,the study of New Physics(NP)beyond the SM has become one of the hot topics in recent years.As is well known,heavy flavour physics is very important in searching for possible signatures of NP.For example,rare B decays and B-B mixing are expected to be good places to search for NP.On the other hand,the non-perturbative nature of Quantum Chromodynamics(QCD)makes theoretical calculations of some flavour pro-cesses quite challenging.Recently,the LHCb collaboration presented measurements of ?b?J/?? and ?b??(2S)? decays,corresponding to 3 fb-1 integrated luminosity recorded at 7 and 8 TeV centre-of-mass energies.The ratio of branching fractions of these two decays is reported as R?b????B(?b??(2S)?)/B(?b?J/?A)=0.513 ± 0.023(stat)± 0.016(syst)± 0.011(B).Both?b?J/?? and ?b??(2S)? decays are b?c(?)s processes at the quark level.However,the two-body non-leptonic b-baryon decays still cannot be calculated from first principle at present.In the literature,evaluation on these processes are based on the factorization hypothesis.In this hypothesis,the hadronic matrix elements are assumed to be product of a decay constant and a form factor,both of which can be calculated in other theoretical frameworks.However,it is noted that the recent LHCb measurements show discrepancies from some theoretical predictions based on the factorization hypothesis.For example,theoretical prediction in the Covariant Quark Model,R?b???=0.8=±0.1,is about 3 ? above the LHCb measurement.In order to achieve more comprehensive understanding of the current discrepancies in ?b??? decays,we focus on phenomenology of the b? c(?)s decays within the SM,which includes the following two parts.On the one hand,by using the most recent theoretical evaluations of the form factors and decay constants,theoretical predictions on RAb??? are analyzed.On the other hand,we have considered other b?c(?)s processes,such as B??cK,B??K,and B??K*decays,and systematically studied the ratios of their branching fractions.This thesis is organized as follows.We give an introduction to flavour physics and its re-cent developments in chapter 1.In chapter 2,we present relevant theoretical basis,such as SM,Operator Product Expansion,Effective Hamiltonian,and helicity amplitude.In chapter 3,the-oretical frameworks for B0??cK0,B+??K+,B+??K*+,and ?b0???0 decays are given.In chapter 4,detailed numerical analyses are presented.Ratios of the branching frac-tions of these four decays are calculated by using the form factors in Relativistic Quark Model,Covariant Quark Model,Light-Cone Sum Rules,and Lattice QCD,while the decay constants obtained from Lattice QCD and experimental data.Comparisons to the current experimental measurements are also given.We conclude in chapter 5.In the appendix,the form factors in various theoretical frameworks are recapitulated.