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

We report on new measurements of Cabibbo-suppressed semileptonic $D^+_{s}$ decays using 3.19fb$^{−1}$ 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 $\mathscr{B}$($D^+_{s}\rightarrow K^{0} e^+\nu_e$) = [ 3.25 ± 0.38 ( stat ) ± 0.16 ( syst ) ] × 10$^{−3}$ and $\mathscr{B}$($D^+_{s}\rightarrow K^{*0} e^+\nu_e$) = [2.37 ± 0.26 (stat) ± 0.20 (syst)] × 10$^{−3}$, which are much improved relative to previous measurements, and the first measurements of the hadronic form-factor parameters for these decays. For $D^+_{s}\rightarrow K^0 e^+\nu_e$, we obtain $f_+$(0) = 0.720 ± 0.084 (stat) ± 0.013 (syst), and for $D^+_{s}\rightarrow K^{*0} e^+\nu_e$, we find form-factor ratios $r_V = V(0)/A_1(0)$ = 1.67 ± 0.34 ( stat ) ± 0.16 (syst) and $r_2$ = $A_2(0)/A_1(0)$ = 0.77 ± 0.28 (stat) ± 0.07 (syst)