Precise measurements of $D^0 \to K^-\ell^+\nu_\ell$ and $D^+ \to \bar K^0\ell^+\nu_\ell$ decays

TL;DR

This paper reports precise measurements of semileptonic D meson decays, including branching fractions, decay asymmetries, and tests for new physics, using data from the BESIII detector at 3.773 GeV.

Contribution

It provides the most precise measurements to date of D meson semileptonic decay parameters and explores potential scalar current contributions beyond the Standard Model.

Findings

Branching fractions measured with high precision.

First determination of forward-backward asymmetries in these decays.

Constraints on scalar current contributions with 1.9 sigma significance.

Abstract

Using $e^+e^-$ collision data corresponding to an integrated luminosity of 20.3 fb$^{-1}$, collected at the center-of-mass energy of 3.773 GeV with the BESIII detector, we present precise measurements of $D^0 \to K^-\ell^+\nu_\ell$ and $D^+ \to \bar K^0\ell^+\nu_\ell$($\ell=e,\mu$) decays. The branching fractions of $D^0\to K^-e^+\nu_e$, $D^0\to K^-\mu^+\nu_\mu$, $D^+\to \bar K^0e^+\nu_e$, and $D^+\to \bar K^0\mu^+\nu_\mu$ are measured to be $(3.527\pm0.005_{\rm stat}\pm0.016_{\rm syst}) \%$, $(3.429\pm0.007_{\rm stat}\pm0.017_{\rm syst}) \%$, $(8.918\pm0.025_{\rm stat}\pm0.050_{\rm syst}) \%$, and $(8.763\pm0.029_{\rm stat}\pm0.052_{\rm syst}) \%$, respectively. The partial decay rates of these four decays are measured with improved precision, and their forward-backward asymmetries are determined for the first time. By performing a simultaneous fit to the measured partial decay rates and the forward-backward asymmetries of $D \to \bar K\ell^+\nu_{\ell}$, a search for a possible scalar current contribution in the $c\to s\ell^+\nu_{\ell}$ transition is performed. The results are ${\rm Re}(c_S^{\mu})=0.007\pm0.008_{\rm stat}\pm0.006_{\rm syst}$ and ${\rm Im}(c_S^{\mu})=\pm(0.070\pm0.013_{\rm stat}\pm0.010_{\rm syst})$, corresponding to a difference from the SM with a significance of $1.9\sigma$. The product of the form factor $f_+(0)$ and the modulus of the $c\to s$ Cabibbo-Kobayashi-Maskawa matrix element $|V_{cs}|$ is determined to be $f_(0)|V_{cs}|=0.7160\pm0.0007_{\rm stat}\pm0.0014_{\rm syst}$. With the inputs $|V_{cs}|=0.97349\pm0.00016$ from the Standard Model global fit or $f_+(0)=0.7452\pm0.0031$ from the lattice quantum chromodynamics calculation, we derive $f_+(0)=0.7355\pm0.0007_{\rm stat}\pm0.0014_{\rm syst}$ and $|V_{cs}|=0.9608\pm0.0009_{\rm stat}\pm0.0019_{\rm syst}\pm0.0040_{\rm LQCD}$. Lepton flavor universality is also tested.