Study Of D_s~+→μ~+ν_μ And D~0→k~-μ~+ν_μ And Test For Lepton Flavor Universality At BESⅢ

Leptonic and semileptonic decays have been well studied in the Standard Model.Their decay rates are proportional to the product of the quark mixing matrix(CKM matrix)ele-ment,which describing the mixing between quark flavor eigenstates and weak interaction eigenstates,and the decay constant or form factor which characterizes the strong interac-tion between initial and final state quarks.The mass of the charm quark is neither light enough for pertubative expansion,nor heavy enough for the application of heavy quark theory,so precision of the theoretical calculations on decay constants and form factors con-cerning charm mesons have always been limited.In recent years,the rapid development of computer resources has led to great improvement of the Lattice quantum chromodynamics(LQCD),which is giving more and more accurate calculations on the decay constants and form factors.As a result,improved study for charm meson leptonic and semileptonic decays is needed to calibrate the nonpertubative QCD calculations and to better determine the quark mixing matrix element with more precise inputs.Meanwhile,in the Standard Model differ-ent generations of lepton should couple with the gauge bosons with same strength,known as lepton flavor universality(LFU).By calculating the ratio of the branching fractions(BFs)to different generations of lepton,most of the uncertainties introduced in the hardonic cur-rent can be cancelled out,making LFU an ideal testbed for the Standard Model.In some new physics models beyond the Standard Model,LFU may be broken,and with the evi-dence of LFU violation observed in recent experiments on B meson semileptonic decays,searching for new physics in this field has drawn lots of attention.Meanwhile,test for LFU in charm meson decays becomes particularly important to check these results and provide more evidence for new physics search.Beijing Electro spectrometer III(BESIII)is a high-performance detector placed at the Beijing Electron Positron Collider(BEPCII).BEPCII works at the energy threshold of τ-charl physics with a design luminosity of 1.0×1033 cm-2s-1,and is the electron positron collider with highest luminosity at this energy region.BESIII started its first run in 2009,and has collected the world’s largest data sample around DD threshold,is a suitable place to study charm meson decays.With a 2.93 fb-1 data sample at center of mass energy 3.773 GeV collected by the BESIII detector,we perform the test for LFU using the decay D0→K-l+vl which has the largest BF in D0 semileptonic decays.The decay Do→K-e+ve has been well studied before,while measurements on D0→K-μvμ still need to be improved.We measure the BF for D0→K-μ+vμ,which is(3.413±0.019stat±0.035syst)%.At the same time the product of the form factor at q2=0 and the quark mixing matrix element |Vcs| is extracted by studying the partial decay rates to be f+D→K(0)|Vcs|=0.7133±0.0038stat±0.0030syst.These results are consistent with previous measurements.With the input of f+D→K(0)calculated by LQCD,we have |Vcs|=0.955±0.005stat± 0.004syst± 0.024LQCD;similarly,with the input of |Vcs| from a global fit of CKM matrix unitarity,we have f+D→K(0)=0.7327±0.0039stat± 0.0030syst.The ratio of the BFs of D0→K-μvμ and D0→K-e+ve is given to be 0.974± 0.007stat± 0.012syst,which is consistent with the theoretical prediction.LFU test is also perfromed in different phase space regions which gives better sensitivity for potential new physics effect and no evidence of violation is found with current statistics.The precision of the quark mixing matrix element measured using D meson semilep-tonic decays is limited by the uncertainty in the form factor expected in theory.On the other hand,recent LQCD calculations on D meson decay constants have achieved the preci-son around 0.2%,which makes D+leptonic decay the best place for measuring |Vcs| in experiment.Using 3.19 fb-1 data taken at center of mass energy 4.178 GeV with the BESIII detector,we get B(Ds+→μ+vμ)=(0.549± 0.016stat± 0.015syst)%,as well as the product of Ds+ decay constant and the quark mixing matrix element fDs+|Vcs|=246.2± 3.6stat± 3.5syst MeV.With the input of fD+ calculated by LQCD,we have |Vcs|=0.985± 0.014stat± 0.014syst;similarly,with the input of |Vcs| from a global fit of CKM matrix unitarity,we have fDs+=252.9± 3.7stat± 3.6syst MeV.These are the best single measuremnts on these constants up to date.With the world average BF of Ds+→τ+vτ,we get B(Ds+→τ+vτ)/B(Ds+→Dμ+vμ)=9.98± 0.52.which is consistent with the Standard Model expectation.