Phenomenological Study On Heavy Hadron Weak Decays

The weak decay of heavy hadrons is one of the ideal places to test the Standard Model of particle physics and to search indirectly for new physics.With the progress on large experimental facilities in recent decades,the theoretical study of heavy flavour physics has entered an era of high precision:the increasing accuracy of experimental measurements also requests more precise theoretical predictions.On the one hand,although the branching ratios and CP violation of numerous decay processes of B mesons have been measured experimentally,there is a paucity of experimental results for the decay of Bs mesons,especially for the quantity ACP,which describes the CP violation,where experimental results are scarce and the measurement errors are large,and there is a lack of theoretical global analysis of the processes of B/Bs meson decays.On the other hand,new heavy hadrons have been continuously searched.For example,in 2017 the LHCb Collaboration discovered the first double heavy baryon Ξcc++ via the decay final state Λc+K-π+π-,and in 2018 the LHCb Collaboration went on to precisely measure its lifetime as τΞcc++=(256-0.022+0.024± 0.014)f s.The use of these new heavy hadron decays to test the Standard Model and probe for new physics requires the development of new theoretical approaches.To address the above issues and challenges,this thesis will discuss and investigate the theoretical and phenomenological aspects of double heavy baryon decays and meson decays at colliders.In this thesis,we first analyse the weak decay processes of the double heavy baryons Ξcc,Ωcc,Ξbc,Ωbc,Ξbb,Ωbb and compare the amplitudes of their decay processes.Based on the SU(3)symmetry of the light quarks,we can write the hadron and the weak decay operators in the form of integrable representations of SU(3)respectively.Using the given SU(3)representation one can obtain the corresponding effective Hamiltonian,which can be expanded to calculate the matrix elements for the double heavy baryon decay.Through the SU(3)symmetry analysis,we derive the relationship between the amplitudes of different decay processes.The results will be useful for the experimental searches for more double heavy baryons,and in combination with the experimental results,the validity of the quark model and the SU(3)symmetry breaking effects can also be checked.In the framework of the light-front quark model,we also analyse the c→d/sl+v,b→ c/ul-v,b→d/sl+l-induced transitions.Both spin 1/2 and spin 3/2 baryons are considered.In the calculations,the form factors in weak decay matrix elements of the the double heavy baryons are calculated using diquark picture and taking into account both scalar diquark and axial vector diquark contributions.Branching ratios and decay widths are predicted with the helicity amplitude method.The decay channels that are most likely to be found experimentally are given after an exhausting consideration of 239 double heavy baryon decay processes.In this thesis,we then adopt the PQCD factorization approach based on the framework of kT factorization for global analysis of the two-body decay of B/Bs mesons.The different decay processes of B/Bs mesons have different kinetic mechanisms,but they all depend on the same non-perturbative parameter:the light-cone distribution amplitude.Thus we analyse the processes from B/Bs mesons to two pseudoscalar mesons,one pseudoscalar meson to one vector meson and two vector mesons,and fit the necessary input parameter to calculate these processes:the Gegenbauer moment in the light-cone distribution amplitude.We also point out the processes that partially require higher order corrections in PQCD.We also fit the CKM phase angle γ with the following value:γ=(75.2±2.9)°.These could be a guide for future work on theoretical calculations and experimental measurements.Following the previous analysis,we study a three-body decay of Bs,Bs→D0π+π-,in the framework of PQCD.In contrast to the two-body decay,the three-body decay has a complex mechanism.The contributions of the resonant states f0(500),f0(980),f0(1500)and f0(1790)in the final states ππ of this process are also considered.The results obtained are also in agreement with those measured experimentally,which is useful for testing the PQCD factorization method and the mechanism of B meson many-body decay.The research in this thesis is closely related to the experiments.For the double heavy baryons,which have not yet been found experimentally,the thesis gives extensive theoretical predictions.For the B/Bs meson decay,which needs to be measured precisely experimentally,the two-body decay process is comprehensively analysed and its three-body decay process is also analysed and calculated precisely.In the future,we will continue this work to pursue more predictive theories in double heavy baryon decays and to explore corrections of their higher-order contributions in B-meson decays.