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

TL;DR

This study provides the first experimental constraints on the scalar current in D meson semileptonic decays, using collision data to measure decay parameters, test lepton flavor universality, and improve precision of key quantities.

Contribution

It presents the first experimental limits on the scalar current in D meson decays and refines measurements of decay rates, form factors, and CKM matrix elements.

Findings

Scalar current parameters deviate from SM by 1.9σ

Branching fractions and form factors are measured with improved precision

Lepton flavor universality is tested across momentum ranges

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 $D^{0(+)}\to \bar K\ell^+\nu_{\ell}$ 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.