Studies On Exclusive Semileptonic Decays Of B_(s) To (1/2)⁺ Excited Charmed Mesons

The semileptonic B(s)decays into charmed mesons are important for extracting the CKM matrix element |Vcb| and searching for new physics beyond the Standard Model.In the last decades,great progress has been made in experiments on the charmed meson spectroscopy,especially the discoveries and measurements of the four lightest excited charmed mesons D0*(2300),D1(2430),D1(2420),D2*(2460)as well as their counterparts of cs states Ds0*(2317),Ds1(2460),Dsl(2536),Ds2*(2573).The jlP=3/2+ doublet mesons(D1(2420),D2*(2460),Ds0*(2317),Ds1(2460))have narrow widths and precisely measured masses,while the jlP=1/2+ doublet mesons(D0*(2300),D1(2430),Ds0*(2317),Ds1(2460))have wide widths and the uncertainties of mass measurements are also relatively large.Most theoretical calculations predict that the production of 3/2+ doublet dominates over that of 1/2+ doublets in semileptonic B decays.Experimental measurements,however,indicate a similar rate of productions.In the present thesis,we aim to study the B(s)to 1/2+ excited charmed meson semileptonic decays in the framework of heavy quark effective field theory(HQEFT).To the next leading order of heavy quark expansion,we calculate the relevant form factors and predict the branching fractions,the lepton flavor universal violation,the polarization asymmetry of lepton,the forward-backward asymmetry of lepton,etc.Firstly,we give a brief review of the quark model and meson spectroscopy,the current status of experiments on B(s)to charmed meson semileptonic decays and the framework of HQEFT.On this basis,we calculate the B(s)?D0*(s0),D1(s1)form factors by using the QCD sum rules,to the next leading order of heavy quark expansion in HQEFT.With the form factor results,we investigate the relevant semileptonic decays B(s)?D0*(s0)(D1(s1))lv,calculating the branching fractions Br,the lepton flavor universal violation observable R,the polarization asymmetry of lepton Pl,the longitudinal polarization fraction of D1(s1)and forward-backward asymmetry of lepton AFB(l).It is found that the values of Br are at the order of 10-4 for the final states with e or ?,while the corresponding results with ? in the final states are at least smaller by one order of magnitude.The uncertainties of Br are about 30%.For l=e,?,the values of Pl are close to-1;for l=?,the values range from-0.3 to-0.1.The results of FL are approximately 0.5.The values of AFB(l)increase with increase of the mass of charged leptons in the final states.For the B(s)?D0*(s0)lv decays,the values of AFB(l)are all positive and close to 0 for the final states with e or ?.In contrast,the corresponding results for the B(s)?D1(s1)lv modes with e or ? in the final states are negative.These results can be tested by more precise experiments in the future.