First Observation Of The Semileptonic Decay D⁺????₁?1270?⁰e+v_e And Measurement Of The Leptonic Decay D_s⁺??⁺v_? At BES?

In the standard model,the strong and weak interactions in semileptonic D decays can be well separated.So,it is easy to explain the process of the semileptonic charm meson decay in theory.The decay rate of semileptonic charm meson decay is proportional to the product of the CKM matrix element and form factor.CKM matrix element describes the mixing between quark weak eigenstates and flavor eigenstates and form factor denotes the strong interaction between the final quarks.Studies of semileptonic D transitions,mediated via c?s(d)l+vl at the quark level,are important for the understanding of nonperturbative strong-interaction dynamics in weak decays.Thirty years ago,quark model has predicted D meson transitions into S wave and P wave.Then,those transitions into S-wave states have been extensively studied in theory and experiment.However,there is still no experimental confirmation of the predicted transitions into P-wave states.The strange axial-vector mesons,K1(1270)0 is the mixture of the 1P1 and 3P1 states with a mixing angle ?K1.ISGW2 quark model implies that the branching fractions of D0(+)?K1(1270)e+ve are about 0.1(0.3)%.Recently,theory predicts that the mixing angle ?K1 may affect the theoretical calculation of the branching fraction of D?K1(1270)e+ve.Even though the mixing angle ?K1 has been studied widely,the value of ?K1 is still very controversial in various phenomenological analyses.Compared with other processes,the semileptonic decay of the final state containing axial-vector particles provides an ideal clean environment with low background for studying the properties of axial-vector particles.Therefore,it is of great significance to study the mixing angle ?K1 by measuring the branching ratio of such decays experimentally.By analyzing a 2.93 fb-1 data sample of e+e-collisions,recorded at a center-of-mass energy of 3.773 GeV with the BES? detector operated at the BEPCII collider,we report the first observation of the semileptonic D+transition into the axial-vector meson D+?K1(1270)0e+ve with a statistical significance of 13.5 ?.Its decay branching fraction is determined to be B[D+?K1(1270)0e+ve]=(2.30 ±0.26-0.21+0.18±0.25)× 10-3,where the first and second uncertainties are statistical and systematic,respectively,and the third originates from the input branching fraction of K1(1270)0?K-?+?0.This results agrees with the CLFQM and LCSR predictions when ?K1?330 or 57°,and rules out the predictions when setting ?K1 negative.The measured branching fraction is 1.4%of the total semileptonic D+decay width,which lies between the ISGW prediction 1%and the ISGW2 prediction 2%.Combining the world averages of the branching fraction of D0?K1(1270)-e+ve and the lifetimes of D0(+),we determine the partial width ratio ?[D+?K1(1270)0e+ve]/?[D0?K1(1270)-e+ve]=1.2±0.7,which is consistent with unit predicted by isospin conservation.In the standard model,the leptonic decays of Ds+?l+ve are described by the annihilation of the initial quark-anti quark pair into a virtual W+.The strong inter-action between the initial quarks can be parameterized as decay constant describing the probability of decay.The decay rate of the leptonic Ds+ decays is proportional to the product of the CKM matrix element and decay constant,|Vcs|fDs+.Measure-ment of the decay constant is important to test the calculation of lattice QCD.In B physics,the decay constant of B meson can not be measured by experiment at present,but it is the input value in some experiment.So,the theoretical calculations of fB is very significant.So,if there are fD+:fDs+:fB,fD+ and fDs+,we can obtain the B meson decay constant fB indirectly.These studies can contribute a lot to the development the theoretical model and improving the precision of calculations.On the other hand,one of the important goals of particle physics is the precise mea-surement and understanding of the CKM matrix elements,fundamental parameters of the standard model.The high precision measurements of all the CKM matrix elements can be used to test the unitarity and normality of CKM matrix.Any significant deviation of measured values from the expectation may indicate the in-completeness of the CKM matrix,thus giving a hint of the existence of new physics.In addition,it is also important to study the ?-? lepton universality by measuring the branching fractions of the leptonic Ds+ decays precisely.By analyzing a data sample of 3.19 fb-1 collected at a center-of-mass energy of 4.178 GeV with the BES? detector,we measure the branching fraction of Ds+??+v? to be(5.22±0.21±0.17)%,via the decay chain of ?+??+v?v?.The measured branching fraction is well consistent with previous measurements from the CLEO-c,Belle,BABAR and BESIII(with other ?+ decay modes)experiments.Combining with the CKM matrix element |Vcs| given by the CKMFitter,the Ds+ decay constant is calculated to be fDS+=250.8 ± 5.0 ± 4.4 MeV.Reversely,combining with the fDs calculated by the latest LQCD,|Vcs| is calculated to be 0.981 ± 0.020 ± 0.017.We also obtain(?)=9.51 ± 0.62 by combining the world averaged value of the branching fraction of Ds+??+v?,that is consistent with the standald model prediction 9.74.So,no evidence of lepton universality violation is found.