Testing lepton flavor universality in terms of BESⅢ and charm-tau factory data

The recent measurements on R_K and R_π imply that there exists a possible violation of the leptonic flavor universality which is one of the cornerstones of the Standard Model. It is suggested that a mixing between sterile and active neutrinos might induce such a violation. In this work we consider the scenarios with one or two sterile neutrinos to explicitly realize the data while the constraints from the available experiments have been taken into account. Moreover, as indicated in literature, the deviation of the real PMNS matrix from the symmetric patterns may be due to a μ-τ asymmetry, therefore the measurements on R_D(Ds)eμ=Γ(D(D_s)→ e⁺ν_e)/Γ(D(D_s)→ μ⁺ν_μ) and R_D(Ds)μτ=Γ(D(D_s)→ μ⁺ν_μ)/Γ(D(D_s)→ τ⁺ν_τ) (and for some other heavy mesons B^± and B_c etc.) may shed more light on the physics responsible for the violation of the leptonic flavor universality. The data of BESⅢ are available to test the universality and that of future charm-tau factories will provide more accurate information. In this work, we will discuss R_D(Ds)eμ and R_D(Ds)μτ in detail and also briefly consider the cases for B^± and B_c.