$SU(3)$ vs. $SU(3) \times SU(3)$ Breaking in Weak Hyperon Decays

Abstract

We consider the predictions of chiral perturbation theory for $SU(3)$ breaking in weak semileptonic and $s$-wave nonleptonic hyperon decays. By defining an expansion sensitive only to $SU(3)$ breaking, we show that the leading corrections give rise to moderate corrections to $SU(3)$ relations (\lsim 20\%), even though the {\it chiral} symmetry $SU(3) \times SU(3)$ appears to be rather badly broken. This explains why $SU(3)$ fits to weak hyperon decays work well even though chiral-symmetry breaking corrections are large. Applying these $SU(3)$-breaking corrections to the analysis of the EMC data, we find that the predicted value of \bra p\mybar s\gamma_\mu\gamma_5 s\ket p is reduced by $\simeq 35\%$, suggesting that the ``EMC effect'' may be less striking than commonly thought.Comment: 15 pages, LBL-33993/CfPA-TH-93-09. (Eliminated multiple fonts in title; some versions of TeX apparently cannot handle this. No change in text.