Phenomenological Study On Weak Decays Of Doubly Heavy Baryons

In the past decades,heavy quark decays have played a very important role in extracting the CKM parameters in the standard model(SM),understanding the mechanism for CP violation,deepening our understanding of dynamics in strong interactions and factorization theorem,and constraining the new physics scenarios beyond SM.This,however,has only made limited use of weak decays of heavy mesons/baryons with a single heavy quark.It is anticipated that weak decays of other heavy hadrons like doubly-heavy baryons will provide equally important information.In 2017,LHCb collaboration has announced the observation of the doubly charmed baryon?cc++ in the ?cc++??c+K-?+?+ decay mode.After this observation,the experimental investiga-tions are being conducted in a number of other decay channels and other doubly heavy baryons.Thus from this viewpoint theoretical studies on weak decays of doubly heavy baryons,not only?cc++,will be of great importance and are highly demanded.Some attempts have been made in some literature,but the aim of this thesis is to provide a comprehensive study.In doing it,we first investigate the 1/2?1/2 decays(here the 1/2 refer to the spin of the initial and final baryons),1/2?3/2 case and flavor-changing-neutral-current induced process.In order to investigate the model dependences,we choose two theoretical approaches:the light-front quark model and QCD sum rules.In the light-front approach,we adopt a quark-diquark picture to calculate the transition form factors.We first consider the 1/2?1/2 transitions,and then the 1/2?3/2 case,which at the quark level are induced by the weak decays of c?d/sl+v or b?u/cl-vv.We derive the analytic results for the form factors and consider both scalar and axial-vector diquarks.The obtained form factors are then applied to predict the partial widths for the semi-leptonic and non-leptonic decays of doubly heavy baryons.We find that a number of branching ratios are sizable and can be examined in future at experimental facilities like LHC,Belle II and CEPC.These results are also useful for a cross-check of ?cc++ and the search for other doubly-heavy baryons.In particular,we obtain the branching fractions:In 2018 the LHCb collaboration has confirmed the existence of ?cc++ through the first channel in the above.Thus it is expected that the latter two channels could also be observed and more information on ?cc++ decays can be probed.In addition,the flavor SU(3)symmetry and symmetry breaking effects in semi-leptonic decays of doubly-heavy baryons are explored.Flavor-changing-neutral-current induced processes are considered as an ideal platform to search for new physics.Baryonic rare decay modes,which are induced by b?d/sl+l-at the quark level,are also important as its mesonic counterparts.Using the form factors we obtained,we analyze the corresponding FCNC induced decay channels,and we find that most of the branching ratios for the b?s processes are 10-8?10-7,while those for the b?d processes are 10-9-10-8.These results are roughly one order of magnitude smaller than those in mesonic sector.This is because the lifetime of the doubly heavy baryon is roughly one order of magnitude smaller than that of B meson.In the framework of QCD sum rules,we first calculate the "decay constants”of doubly heavy baryons ?cc,?cc,?bb,?bb,?bc and ?bc.In the analysis we include both the positive and negative parity baryons,and find a less severe 1/2-contamination.Results for decay constants are ingredients for the study of weak decays and other properties of doubly heavy baryons.Then we present a QCD sum rules analysis of the form factors of doubly heavy baryons to singly heavy baryons.We include the perturbative contribution and condensation contributions up to dimension 5.We also evaluate part of contributions from the gluon-gluon condensate,and find that these contributions are negligible.These form factors are then used to study the semi-leptonic and non-leptonic decays.Future experimental measurements can examine these predictions and test the validity of applying the light-front approach and QCD sum rules to doubly heavy baryons.It is anticipated that better understanding on doubly-heavy baryons can be achieved in future.