First Observation Of D~+→ημ~+v_μ And Study Of Its Decay Dynamics At BESⅢ

By analyzing a data sample of 2.93 fb-1 collected at the center-of-mass en-ergy of 3.773 GeV with the BESIII detector,we have studied the semi-muonic decay D+→ημ+υμ for the first time.The absolute branching fraction is deter-mined to be B(D+→ημ+υμ)=(10.4±1.0stat.±0.5syst.)×10-4.Combining the world average of B(D+→ηe+υe)=(11.4±1.0)×10-4,we ob-tain the ratio of the two branching fractions to be 0.91±0.13,which agrees with the lepton universality of Standard Model within uncertainty.By analyzing the differential decay rates of D+→ημ+υμ,dΓ/dq2 determine the product of the hadronic form factor f+η(0)and the CKM matrix element |Vcd| to be f+η(0)|Vcd|=0.087±0.008stat.±0.002syst..Taking |Vcd|=0.22438±0.00044 from the CKMFitter,we obtain f+η(0)=0.39±0.04stat.±0.01syst..These results can be used to validate various theoretical calculations of branching fraction and hadronic form factor for D+→ημ+vμ.