First Measurement of the Form Factors in ${D}_{s}^{+}\to {K}^{0}{e}^{+}{\nu }_{e}$ and ${D}_{s}^{+}\to {K}^{*0}{e}^{+}{\nu }_{e}$ Decays

We report on new measurements of Cabibbo-suppressed semileptonic ${D}_{s}^{+}$ decays using $3.19\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ annihilation data sample collected at a center-of-mass energy of 4.178 GeV with the BESIII detector at the BEPCII collider. Our results include branching fractions $B\left({D}_{s}^{+}\to {K}^{0}{e}^{+}{\nu }_{e}\right)=\left[3.25±0.38\left(\mathrm{stat}\right)±0.16\left(\mathrm{syst}\right)\right]×{10}^{-3}$ and $B\left({D}_{s}^{+}\to {K}^{*0}{e}^{+}{\nu }_{e}\right)=\left[2.37±0.26\left(\mathrm{stat}\right)±0.20\left(\mathrm{syst}\right)\right]×{10}^{-3}$, which are much improved relative to previous measurements, and the first measurements of the hadronic form-factor parameters for these decays. For ${D}_{s}^{+}\to {K}^{0}{e}^{+}{\nu }_{e}$, we obtain ${f}_{+}\left(0\right)=0.720±0.084\left(\mathrm{stat}\right)±0.013\left(\mathrm{syst}\right)$, and for ${D}_{s}^{+}\to {K}^{*0}{e}^{+}{\nu }_{e}$, we find form-factor ratios ${r}_{V}=V\left(0\right)/{A}_{1}\left(0\right)=1.67±0.34\left(\mathrm{stat}\right)±0.16\left(\mathrm{syst}\right)$ and ${r}_{2}={A}_{2}\left(0\right)/{A}_{1}\left(0\right)=0.77±0.28\left(\mathrm{stat}\right)±0.07\left(\mathrm{syst}\right)$.