Semileptonic Hyperon Decays

We review the status of hyperon semileptonic decays. The central issue is the $V_{us}$ element of the CKM matrix, where we obtain $V_{us}=0.2250(27)$. This value is of similar precision, but higher, than the one derived from $K_{l3}$, and in better agreement with the unitarity requirement, $\mathrm{\mid }{V}_{ud}{\mathrm{\mid }}^2+\mathrm{\mid }{V}_{us}{\mathrm{\mid }}^2+\mathrm{\mid }{V}_{ub}{\mathrm{\mid }}^2=1$. We find that the Cabibbo model gives an excellent fit of the existing form factor data on baryon beta decays ($\chi^{2} = 2.96$for3degreesoffreedom)with$F + D = 1.2670 \pm 0.0030$,$F - D = -0.341 \pm 0.016$, and no indication of flavour-SU(3)-breaking effects. We indicate the need of more experimental and theoretical work, both on hyperon beta decays and on $K_{l3}$ decays.