Semileptonic Hyperon Decays and CKM Unitarity

Using a technique that is not subject to first-order SU(3) symmetry breaking effects, we determine the $V_{us}$ element of the CKM matrix from data on semileptonic hyperon decays. We obtain $V_{us}$ =0.2250(27). This value is of similar precision to the one derived from $K_{l3}$, but higher and in better agreement with the unitarity requirement, $\mathrm{\mid }{V}_{ud}{\mathrm{\mid }}^2+\mathrm{\mid }{V}_{us}{\mathrm{\mid }}^2+\mathrm{\mid }{V}_{ub}{\mathrm{\mid }}^2=1$.