Investigating the ratio of CKM matrix elements $|V_{ub}|/|V_{cb}|$ from semileptonic decay $B_s^0\to K^-\mu^+\nu_\mu$ and kaon twist-2 distribution amplitude

In this paper, we calculate the ratio of Cabibbo-Kobayashi-Maskawa matrix elements, $|V_{ub}|/|V_{cb}|$, based on the semileptonic decay $B_s^0\to K^-\mu^+\nu_\mu$. Its key component, the $B_s\to K$ transition form factor $f^{B_s\to K}_+(q^2)$, is studied within the QCD light-cone sum rules approach by using a chiral correlator. The derived $f^{B_s\to K}_+(q^2)$ is dominated by the leading-twist part, and to improve its precision, we construct a new model for the kaon leading-twist distribution amplitude $\phi_{2;K}(x,\mu)$, whose parameters are fixed by using the least squares method with the help of the moments calculated by using the QCD sum rules within the background field theory. The first four moments at the initial scale $\mu_0 = 1~{\rm GeV}$ are, $\langle \xi^1\rangle _{2;K} = -0.0438^{+0.0053}_{-0.0075}$, $\langle \xi^2\rangle _{2;K} = 0.262 \pm 0.010$, $\langle \xi^3\rangle _{2;K} = -0.0210^{+0.0024}_{-0.0035}$ and $\langle \xi^4\rangle _{2;K} = 0.132 \pm 0.006$, respectively. And their corresponding Gegenbauer moments are, $a^{2;K}_1 = -0.0731^{+0.0089}_{-0.0124}$, $a^{2;K}_2 = 0.182^{+0.029}_{-0.030}$, $a^{2;K}_3 = -0.0114^{+0.0008}_{-0.0016}$ and $a^{2;K}_4 = 0.041^{-0.003}_{+0.005}$, respectively. At the large recoil region, we obtain $f^{B_s\to K} _+ (0) = 0.270^{+0.022}_{-0.030}$. By extrapolating $f^{B_s\to K}_+(q^2)$ to all the physical allowable region, we obtain a $|V_{ub}|$-independent decay width for the semileptonic decay $B_s^0\to K^-\mu^+\nu_\mu$, $5.626^{+1.271}_{-0.864} \times 10^{-12}\ {\rm GeV}$, which then leads to $|V_{ub}|/|V_{cb}| = 0.072\pm0.005$.