First measurement of the form factors in $D^+_{s}rightarrow K^0 e^+ u_e$ and $D^+_{s}rightarrow K^{*0} e^+ u_e$ decays

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 $mathcal B({D^+_srightarrow K^0 e^+ u_{e}})=(3.25pm0.38({rm stat.})pm0.16({rm syst.}))times10^{-3}$ and $mathcal B({D^+_srightarrow K^{*0} e^+ u_{e}})=(2.37pm0.26({rm stat.})pm0.20({rm syst.}))times10^{-3}$ which are much improved relative to previous measurements, and the first measurements of the hadronic form-factor parameters for these decays. For $D^+_srightarrow K^0 e^+ u_{e}$, we obtain $f_+(0)=0.720pm0.084({rm stat.})pm0.013({rm syst.})$, and for $D^+_srightarrow K^{*0} e^+ u_{e}$, we find form-factor ratios $r_V=V(0)/A_1(0)=1.67pm0.34({rm stat.})pm0.16({rm syst.})$ and $r_2=A_2(0)/A_1(0)=0.77pm0.28({rm stat.})pm0.07({rm syst.})$.