SEMILEPTONIC HYPERON DECAYS

We review the status of hyperon semileptonic decays. The central issue is the Vus element of the CKM matrix, for which we obtain Vus = 0.2250 (27). This value is similar in precision to the one derived from Kl3, but higher, and in better agreement with the unitarity requirement, |Vud|² + |us|² + |Vub|² = 1. We find that the Cabibbo model gives an excellent fit to baryon–beta-decay form-factor data (χ² = 2.96 for 3 degrees of freedom) with F + D = 1.2670 ± 0.0030, F − D = −0.341 ± 0.016, and no indication of flavor-SU(3)–breaking effects. We indicate the need for more experimental and theoretical work, both on hyperon beta decays and on Kl3 decays.