Semileptonic Hyperon Decays and Cabibbo-Kobayashi-Maskawa Unitarity

Using a technique that is not subject to first-order SU(3) symmetry breaking effects, we determine the $V_{us}$ element of the Cabibbo-Kobayashi-Maskawa matrix from data on semileptonic hyperon decays. We obtain $V_{us}=0.2250(27)$, where the quoted uncertainty is purely experimental. This value is of similar experimental precision to the one derived from $K_{l3}$, but it is higher and thus in better agreement with the unitarity requirement, $|V_{ud}{|}^2+|V_{us}{|}^2+|V_{ub}{|}^2=1$. An overall fit, including the axial contributions and neglecting SU(3) breaking corrections, yields $F+D=1.2670\pm 0.0035$ and $F-D=-0.341\pm 0.016$ with ${\chi }^2=2.96/3\mathrm{\text{degrees of freedom}}$.