Constraining λbs by Bs →γ∗ and Bs →φ form factors

We calculate the form factors FV(q2,q′2) and FTV(q2,q′2) describing the Bs→γ∗ transition induced by the vector and tensor weak currents in a broad range of values of q2 and q′2 far below the quark thresholds in both q2 and q′2 channels. These form factors are calculated via the distribution amplitudes of the Bs-meson. We then interpolate the obtained results by a formula that contains pole at q′2=Mφ2 and extract the residue which gives the Bs→φ transition form factors V(q2) and T1(q2). In this way we obtain theoretical predictions for these form factors without invoking quark-hadron duality and QCD sum rules. Furthermore, we calculate the relationship between V(0) and T1(0) and the parameter λBs(μ), the inverse moment of the Bs-meson distribution amplitude. Using the available predictions for V(0) and T1(0) coming from approaches not referring to the Bs-meson distribution amplitudes, we obtain the estimate λBs(μ≃mb)=(0.62±0.10) GeV.