Flavor SU (3) symmetry breaking in the hyperon semileptonic decay form factors is analyzed using the 1/N c expansion. A detailed comparison with experimental data shows that corrections to f 1 are approximately 10%, which agrees with theoretical expectations. Corrections to g 1 are compatible with first-order symmetry breaking. A fit to the experimental data allows one to predict the g 1 form factor for Ξ 0 → Σ + decay. The proton matrix element of the T 8 component of the axial current (which is equal to 3F − D in the SU (3) symmetry limit) is found to be ≈ 0.34-0.46.

The baryon axial vector current is computed at one-loop order in heavy baryon chiral perturbation theory in the large-Nc limit, where Nc is the number of colors. Loop graphs with octet and decuplet intermediate states cancel to various orders in Nc as a consequence of the large-Nc spin-flavor symmetry of QCD baryons. These cancellations are explicitly shown for the general case of N f flavors of light quarks. In particular, a new generic cancellation is identified in the renormalization of the baryon axial vector current at one-loop order. A comparison with conventional heavy baryon chiral perturbation theory is performed at the physical values Nc = 3, N f = 3.

We show how to compute loop graphs in heavy baryon chiral perturbation theory including the full functional dependence on the ratio of the Delta-nucleon mass difference to the pion mass, while at the same time automatically incorporating the 1/N c cancellations that follow from the large-N c spin-flavor symmetry of baryons in QCD. The one-loop renormalization of the baryon axial vector current is studied to demonstrate the procedure. A new cancellation is identified in the one-loop contribution to the baryon axial vector current. We show that loop corrections to the axial vector currents are exceptionally sensitive to deviations of the ratios of baryon-pion axial couplings from SU (6) values.

We obtain an expression for the Dalitz plot of semileptonic decays of polarized hyperons including radiative corrections to order ␣ and neglecting terms of the order ␣q/M 1 , where q is the four-momentum transfer and M 1 is the mass of the decaying hyperon. Our results are specialized to exhibit the angular correlation between such polarization and the momentum of the emitted hyperon. The model dependence of radiative corrections is kept in a general form within this approximation which is suitable for model-independent experimental analysis. Our final result, valid for charged as well as for neutral hyperons, allows us to obtain the angular spinasymmetry coefficient of the emitted hyperon. ͓S0556-2821͑97͒04709-7͔

A model-independent determination of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix (CKM) element V us from five measured strangeness-changing hyperon semileptonic decays is performed. Flavor SU3 symmetry breaking effects in the leading vector and axial-vector form factors are analyzed in the framework of the 1=N c expansion of QCD. A fit to experimental data allows one to extract the value V us 0:2199 0:0026, which is comparable to the one from K e3 decays. This reconciliation is achieved through second-order symmetry breaking effects of a few percent in the form factors f 1 , which increase their magnitudes over their SU3 predictions.