Semileptonic Hyperon Decays

Abstract

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, $|V_{ud}{|}^2+|V_{us}{|}^2+|V_{ub}{|}^2=1$. We find that the Cabibbo model gives an excellent fit of the existing form factor data on baryon beta decays (${\chi }^2=2.96$ for 3 degrees of freedom) 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.

Comments: 37 pages, 8 figures, 4 tables, Final version of this material is scheduled to appear in the Annual Review of Nuclear and Particle Science Vol. 53