First Experimental Constraint on the Scalar Current in the $D^{0(+)}\to \bar{K}{\ell }^{+}\hat{I}{½}_{\ell }$ Transition

Abstract

Using 20.3 fb${}^{-1}$ of $e^{+}{e}^{-}$ collision data taken at the center-of-mass energy of $\sqrt{s}=3.773$ GeV, we report the first experimental constraint on the scalar current in the $D^{0(+)}\to \bar{K}{\ell }^{+}\hat{I}{½}_{\ell }$ transitions, based on a simultaneous fit to the first measured forward-backward asymmetries and precisely determined partial decay rates. The parameters of the scalar current are determined to be $\mathrm{Re}(c_S^{\hat{I}}¼)=0.017\pm 0.008_{\mathrm{stat}}\pm 0.006_{\mathrm{syst}}$ and $\mathrm{Im}(c_S^{\hat{I}}¼)=\pm (0.077\pm 0.011_{\mathrm{stat}}\pm 0.009_{\mathrm{syst}})$, which deviates from the SM by a significance of $2.3\ddot{I}ƒ$. The branching fractions of $D^{0(+)}\to \bar{K}{\ell }^{+}\hat{I}{½}_{\ell }$, the hadronic form factor $f_{+}(0)$, and the modulus of the $c\to s$ CKM matrix element $|V_{cs}|$ are also determined with improved precision. In addition, lepton flavor universality is tested via the ratios of the decay rates between semi-muonic and semi-electronic decays in the full momentum transfer range and in subranges.

11 pages, 3 figures, submitted to PRL