Heavy Hadron Weak Decays With Non-perturbative Approaches

In particle physics,weak decays of heavy hadrons provide an ideal platform for precisely testing the Standard Model,understanding the CP asymmetries and searching for new physics.The semileptonic W-emission decay of heavy hadrons can be used to extract the CKM matrix elements:V_cb,V_ub,V_cs,V_cd,while the flavor-changing-neutral-current induced decays of bottom hadrons are valuable to constrain the new physics.By exploring the decay branching ratios,final state angular distribution,forward–backward asymmetry and spin polarization on both experimental and theoretical side,one can quantify the consistency between experimental results and the standard model predictions,which gives important constraints on new physics effects.Theoretically,the main difficulty in the study of heavy meson decays is to reliably calculate the hadron transition matrix element.At the hadronic scale,the coupling constant of strong interactions is large which prohibits the use of perturbative expansions inα_s.In some processes factorization scheme might be adopted to separate the short-distance and long-distance degrees of freedom,but until now only a few of decay processes have been proved to be factorizable.In addition,such proof is only conducted at the leading power expansion in 1/Q with Q being the hard-scattering energy.In practice,there are a few non-perturbative approaches to handle weak decays of heavy flavored hadrons.These include light-cone QCD sum rules,chiral perturbation theory,flavor SU（3）symmetry analysis,light-front quark model as well as lattice QCD.In this thesis,we will systematically explore scalar,vector and tensor form factors of two light mesons within the chiral perturbation theory（χPT）.We first calculate these form factors at the one-loop accuracy withinχPT and extend the applicable region up to 1 GeV using the unitarized approach.With these form factors at hand,we study several multi-body decays of heavy mesons including the D→ππ?（?）,D_s→π⁺π^-?νand B_s→π⁺π^-?⁺?^-/（?）ν.All these decays contain contribution of an mediate S-wave two-light-meson resonant.We calculate the hadronic transition matrix elements using light-cone QCD sum rules,while the final interaction among the two mesons is described by scalar form factors.We derive a general angular distribution which can be used to extract branching ratios,forward–backward asymmetry and other physical observables.By comparing the scalar form factors derived from unitarized chiral perturbation theory with those fitted by B_s→J/ψπ⁺π^-data,we find that theππmass distribution from these two approaches are consistent with each other.Predictions on differential decay widths are consistent with the current experimental measurements and could be examined more precisely at BES-III,LHCb and Super-B factory in the future.Another part of this thesis is devoted to study weak decays of doubly heavy baryons.We use the flavor SU（3）symmetry to simplify the independent decay amplitudes and adopt the light-cone QCD sum rules to calculate the individual form factors.Through the SU（3）analysis,we derive a number of relations between the decay widths for various channels.In the light-cone sum rules calculation,we concentrate on the doubly-heavy baryons decays intoΛ_b,cbaryons.We compute the baryon transition form factors,and find that the f₁and g₁are at the same order of magnitude.Our predictions of decay widths and branching ratios are consistent with those given in the literatures,and are waiting for examination at future experiments.Without any doubt,heavy flavor physics will still play an important role in particle physics in the next few decades.We expect in the future the experimental facilities at the LHC,BEPC and Super-B will perform more analyses of weak decays of heavy hadrons.On the theoretical side,it is demanded to simultaneously improve the precision of calculation and develop new effective tools.