Title: First Measurement of the $D_s^+\rightarrow K^0μ^+ν_μ$ Decay Show Chinese title

Title: 首次测量$D_s^+\rightarrow K^0μ^+ν_μ$衰变

Abstract: We report the first measurement of the semileptonic decay $D^+_s \rightarrow K^0\mu^+\nu_{\mu}$, using a sample of $e^+e^-$ annihilation data corresponding to an integrated luminosity of $7.33~\mathrm{fb}^{-1}$ collected at center-of-mass energies between 4.128 to 4.226~GeV with the BESIII detector at the BEPCII collider. The branching fraction of the decay is measured to be $\mathcal{B}(D^+_s\rightarrow K^0\mu^+\nu_{\mu}) = (2.89 \pm 0.27_{\rm stat} \pm 0.12_{\rm syst})\times 10^{-3}$, where the first uncertainty is statistical and the second is systematic. Based on a simultaneous fit to the partial decay rates in $q^2$ intervals measured in $D^+_s \rightarrow K^0\mu^+\nu_{\mu}$ and $D^+_s \rightarrow K^0e^+\nu_{e}$ decays, the product value of the form factor $f^{K^0}_{+}(0)$ and the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cd}|$ is measured to be $f^{K^0}_{+}(0)|V_{cd}|=0.140\pm0.008_{\rm stat}\pm0.002_{\rm syst}$. Using $|V_{cd}|=0.22486\pm0.00068$ as an input, the hadronic form factor is determined to be $f^{K^0}_{+}(0)=0.623\pm0.036_{\rm stat} \pm 0.009_{\rm syst}$ at $q^2=0$. This is the most precise determination of $f^{K^0}_{+}(0)$ in the $D^+_s \rightarrow K^0$ transition to date. The measured branching fraction and form factor presented in this work provide the most stringent test on various non-perturbative theoretical calculations. Taking $f^{K^0}_{+}(0)=0.6307\pm0.0020$ from lattice calculations as an input, we obtain $|V_{cd}|=0.220\pm0.013_{\rm stat}\pm0.003_{\rm syst}\pm0.001_{\rm LQCD}$, which is the most precise determination of $|V_{cd}|$ using the $D_s^+\rightarrow K^0\ell^+\nu_{\ell}$ decays. In addition, lepton flavor universality is tested for the first time with $D^+_s \rightarrow K^0\ell^+\nu_{\ell}$ decays in full and separate $q^2$ intervals. No obvious violation is found. Show Chinese abstract

Abstract: 我们报告了首次对半轻子衰变$D^+_s \rightarrow K^0\mu^+\nu_{\mu}$的测量，使用了一个对应于积分亮度为$7.33~\mathrm{fb}^{-1}$的$e^+e^-$粒子对湮灭数据样本，在 BEPCII 对撞机的 BESIII 探测器上于质心系能量在 4.128 到 4.226~GeV 之间收集。 该衰变的分支比被测量为$\mathcal{B}(D^+_s\rightarrow K^0\mu^+\nu_{\mu}) = (2.89 \pm 0.27_{\rm stat} \pm 0.12_{\rm syst})\times 10^{-3}$，其中第一个不确定性是统计的，第二个是系统性的。 基于对在$q^2$区间内测得的$D^+_s \rightarrow K^0\mu^+\nu_{\mu}$和$D^+_s \rightarrow K^0e^+\nu_{e}$衰变的分支率的同时拟合，形式因子$f^{K^0}_{+}(0)$与Cabibbo-Kobayashi-Maskawa矩阵元$|V_{cd}|$的乘积值被测量为$f^{K^0}_{+}(0)|V_{cd}|=0.140\pm0.008_{\rm stat}\pm0.002_{\rm syst}$。 使用$|V_{cd}|=0.22486\pm0.00068$作为输入，强子形式因子被确定为$f^{K^0}_{+}(0)=0.623\pm0.036_{\rm stat} \pm 0.009_{\rm syst}$在$q^2=0$。 这是迄今为止在$D^+_s \rightarrow K^0$转变中对$f^{K^0}_{+}(0)$的最精确确定。 本文中测量的分支比和形式因子提供了对各种非微扰理论计算最严格的检验。 将格点计算中的$f^{K^0}_{+}(0)=0.6307\pm0.0020$作为输入，我们得到$|V_{cd}|=0.220\pm0.013_{\rm stat}\pm0.003_{\rm syst}\pm0.001_{\rm LQCD}$，这是利用$D_s^+\rightarrow K^0\ell^+\nu_{\ell}$衰变对$|V_{cd}|$的最精确测定。 此外，首次在完整的和单独的$q^2$区间内对$D^+_s \rightarrow K^0\ell^+\nu_{\ell}$衰变进行了轻子味普遍性的测试。 未发现明显的违反情况。