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 $D^{ 0(+)}\to \bar K\ell^+\nu_{ \ell}$ 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 ${ \rm Re}(c_S^{ \mu})=0.007\pm0.008_{ \rm stat}\pm0.006_{ \rm syst}$ and ${ \rm Im}(c_S^{ \mu})=\pm(0.070\pm0.013_{ \rm stat}\pm0.010_{ \rm syst})$, which deviates from the SM by a significance of $1.9\sigma$. The branching fractions of $D^{ 0(+)}\to \bar K\ell^+\nu_\ell$, 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.
Categories: hep-ex