Isospin breaking in neutronβdecay and SU(3) violation in semileptonic hyperon decays

Present precision measurements of the neutron lifetime lead to a CKM matrix element |Vud|, which is 3 standard deviations off the value inferred from heavy quark decays, etc. We investigate the possibility whether isospin-breaking effects in the neutron-to-proton vector current transition matrix element 〈p|V₀⁺|n〉=1+δgV could eventually close this gap. For that we calculate in chiral perturbation theory the effect of pion and kaon loops on the matrix element 〈p|V₀⁺|n〉 taking into account the mass differences of the charged and neutral mesons. We find a negligibly small isospin-breaking effect of δgV-4×10^-5. The crucial quantity in the analysis of neutron β-decay precision measurements is thus the radiative correction term ΔR. Furthermore, we calculate in heavy baryon chiral perturbation theory the SU(3) breaking effects on the vector transition charges of weak semileptonic hyperon decays. We find for these quantities channel-dependent relative deviations from the SU(3) limit which range from -10% to +1%.