Study On The Weak Decays Of Heavy Flavor Hadrons

The heavy flavor physics plays an important role in testing the Standard Model (SM)and fixing its parameters, as well as probing the New Physics beyond the SM. During thelast two decades, it has been a hot topic in the study of particle physics on both experi-mental and theoretical aspects. It is not only because that its study needs a new theoreticalframework, but also that abundant dynamical mechanisms are embedded in various de-cays of the heavy flavor hadrons as well as that it is a perfect ground for the study ofthe nonperturbative effect of quantum chromodynamics (QCD). This thesis aims to studysome weak decay processes of the heavy flavor hadrons and are mainly about the threetopics as follows.First, The inclusive B→X_c(?)(V|-) and exclusive B→D(*)(?)(V|-) semileptonic processesarethetwoindependentwaysofdeterminingthevalueoftheCabibbo-Kobayashi-Maskawa(CKM) matrix element∣V_cb∣ and they can provide verifications for each other. However,thevaluesofV_cb∣thatdeterminedbythemcanberoughlycompatiblewitheachotherwiththe precision of the one from the inclusive process a little higher. In order to reduce theerrors in the exclusive processes and understand the exclusive components of the inclu-sive process, we study the semileptonic decays of the ground state B meson into orbitally??-wave and ??-wave excited charmed mesons under the framework of heavy quark effec-tive theory (HQET). We employ the QCD sum rule method to estimate the correspondingIsgur-Wise functions and then we predict the decay widths and branching ratios of theseprocesses. We have also discussed the choices of the interpolating currents for the heavy-lightmesonsinHQETandfindthatwhenthespin-parityofthelightdegreeoffreedomareS_l^p=(1+)/2,(3-)/2,and (5+)/2, we should choose the currents with derivative of one order higher.With the angular momentum of final mesons increased, the dynamics depressed effect inthe semileptonic B decays into them are more and more remarkable.Second, many new D_s and B_s excited states have been observed in experiments dur-ing the last few years and the theoretical study on them has been a hot topic. We investi-gate systematically the semileptonic B_s^**decays into several low-lying doublets of the D_smesons in this thesis. These processes are studied with HQET combined with QCD sumrules. We have calculated the semileptonic decay widths and branching ratios of these four orbitally ??-wave excited mesons into the H, S, and T doublets of the D_s mesons.Our results shows that, the branching ratios of these processes are tiny and therefore hardto be observed in experiments,but some of them are possible to be observed in the nearfuture. The heavy quark symmetry (HQS) predicts that the corresponding leading orderIsgur-Wise functionξ(y) which parameterizes the hadonic matrix elements of the weakcurrents between the H doublet of B mesons and the same doublet of D mesons satisfiesξ(y = 1) = 1. We have shown that similar conclusions can also stand for the S andT doublets, that is, the corresponding Isgur-Wise functions also satisfy the normalizationcondition at the zero recoil point.Finally,thestudyofheavyflavorbaryonsisanothersub-fieldinheavyflavorphysics.The dynamical information is embedded in various decay processes among which therare radiative and rare semileptonic decays of the heavy flavor baryons are importantones. These processes happen via the flavor-changing neutral current (FCNC) and aredepressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism. We investigate the pro-cessesΛ_b→Λ_γ, andΛ_b→Λl+l^- with light-cone QCD sum rules in full QCD and predicttheir widths and branching ratios. We have considered both CZ-type and Ioffe-type cur-rents for theΛ?? baryon and find that the choice of the interpolating current will affect theresult dramatically, especially for the rare radiative process. In addition, We study the rareradiative decays of the baryonsΣ_b andΞ_b with the same approach and predict their decaywidths and branching ratios.