We report on new measurements of Cabibbo-suppressed semileptonic $D_s^{+}$ decays using $3.19{\mathrm{fb}}^{-1}$ 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 $\mathcal{B}(D_s^{+}\to K^0{e}^{+}{\nu }_e)=[3.25\pm 0.38(\mathrm{stat})\pm 0.16(\mathrm{syst})]\times {10}^{-3}$ and $\mathcal{B}(D_s^{+}\to K^{*0}{e}^{+}{\nu }_e)=[2.37\pm 0.26(\mathrm{stat})\pm 0.20(\mathrm{syst})]\times {10}^{-3}$, 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 $f_{+}(0)=0.720\pm 0.084(\mathrm{stat})\pm 0.013(\mathrm{syst})$, and for $D_s^{+}\to K^{*0}{e}^{+}{\nu }_e$, we find form-factor ratios $r_V=V(0)/A_1(0)=1.67\pm 0.34(\mathrm{stat})\pm 0.16(\mathrm{syst})$ and $r_2=A_2(0)/A_1(0)=0.77\pm 0.28(\mathrm{stat})\pm 0.07(\mathrm{syst})$.

• Received 8 November 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.061801

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Published by the American Physical Society