Characterize the properties of $D_s^+$-meson decay constant and leptonic decays by using QCD sum rules within background field theory framework
The leptonic decays of the $D_s^+$-meson have received considerable attention in recent years. In this work, we perform a precise calculation of the decay constant $f_{D_s^+}$ using the QCD sum rules method within the background field theory framework. In our calculation, we fully include the quark propagator contributions up to dimension-six condensates. By adopting two different constraint schemes, we obtain $f_{D_s^+}^{\text{(I)}} = 253.0_{-3.1}^{+3.3}\ \text{MeV}$ and $f_{D_s^+}^{\text{(II)}} = 251.8_{-1.3}^{+1.4}\ \text{MeV}$, respectively, both of which are in good agreement with existing theoretical and experimental results. The conventional scheme follows the standard Borel window criteria, while the derivative scheme reduces the dependence of the decay constant on the Borel parameter through an auxiliary function. Based on these decay constants and incorporating the NLO electroweak radiative corrections, we further calculate the branching fractions for the three leptonic decay channels for both schemes. Combined with the latest branching fraction $\mathcal{B}(D_s^+ \to \mu^+ \nu_\mu)$ from the PDG, we extract the CKM matrix elements $|V_{cs}|^{\text{(I)}} = 0.967 \pm 0.012$ and $|V_{cs}|^{\text{(II)}} = 0.970 \pm 0.005$ from the two schemes, respectively.