Phenomenological Study On Semi-leptonic Weak Decays Of Light And Doubly Heavy Baryons

Semi-leptonic weak decays of light and heavy baryons provide a unique probe for testing the standard model of particle physics,hunting for new physics beyond the standard model and understanding quantum chromodynamic factorization.It has been one of the frontier topics in the field of particle physics research.In 2017,the LHCb collaboration announced the observation of the doubly charmed baryon (?)cc++ in the Λc+K-π+π+ final state,and the mass is measured as m(?)cc++(3621.40±0.72±0.27±0.14)MeV.In 2018,the lifetime of (?)cc++ has been measured asτ(?)cc++=(256-0.022+0.024±0.014)fs.These experimental studies give a new platform for the study of weak interactions.Based on the large amount of data accumulated by different experiments in future,it is expected that other doubly-heavy baryons can be found at LHC and other experiment.Since the lowest-lying doubly-heavy baryons are stable against strong interactions,theoretical studies of their weak decays are of great importance.In this thesis,we first study the decay process induced by the flavor-changing neutral current s→dvv of light baryons(hyperons)in standard model and beyond.We point out that within the framework of standard model,the branching ratios of these semi-leptonic weak decay processes are at the order 10-14～10-11.The transition form factors can introduce about 5%～10%uncertainties.In new physics models,we find that branching ratios can be enhanced by a factor 2～7.These theoretical results can be examined by the experimental studies on BESIII and other relevant experiments,and then can be used to constrain the new physics.The second part of this thesis is denoted to the theoretical study on input parameters for weak decays of doubly-heavy baryons.Using the QCD sum rules,we explore the "decay constants" and masses of the doubly-heavy baryons.Aside from the contributions from lowest-lying states with JP=1/2+,the contributions from negative parity baryons with JP=1/2-are also included.We find that the result of m(?)cc++ is consistent with the experimental results from the LHCb collaboration when the errors are taken into account.Compared to the case only considering the ground state baryons with JP=1/2+,the contributions from negative parity baryons are not severe and the numerical results of the decay constants are stable.These "decay constants" and masses can be used as inputs for investigating the semi-leptonic weak decays and other properties of the doubly-heavy baryons.For the weak decays of doubly-heavy baryons induced by charged current,we then calculate the form factors for the transition from doubly-heavy baryons (?)QQ’ into the sextet ΣQ’ within light-cone sum rules.Using these form factors and "decay constants",we give predictions for partial decay widths and branching ratios of the corresponding semi-leptonic processes.It can be found that the branching ratios of (?)cc++→Σc+l+vl and (?)cc+→c0l+vl are at the order of 10-3～10-2,which can be examined in future at experimental facilities.In the last part of this thesis we conduct a systematic study of semi-leptonic weak decays of doubly-heavy baryons in the light-front quark model.The spin-1/2 or spin-3/2 final baryons are considered and both the charged current and the flavor changing neutral current are systematically studied.At the quark level,these decay processes are induced by the c →d/sl+v,b c/ul-v and b→d/sl+l-.We calculate these transitions form factors within the light-front quark model,and provide a new approach to derive the flavor-spin overlapping factors using the flavor SU(3)symmetry.With the obtained transition form factors and flavor-spin overlapping factors,we perform a phenomenological study of the corresponding semi-leptonic decays of doubly-heavy baryons,and 239 decay channels are studied.We find that most branching ratios for the semi-leptonic decays induced by the c→d/sl+v transitions are at the order of 10-3～10-2,which might be useful for the search of other doubly-heavy baryons.These theoretical predictions can be examined at the experimental facilities in the future,and the applicability of these approaches,QCD sum rules,light-cone sum rules and light-front quark model can be then tested.Through these studies,we also expect a deeper understanding of the dynamics of semi-leptonic weak decays of doubly-heavy baryons.