Quark Helicity Research Articles

QCD calculation of the parity-violating NNπ amplitude in the Weinberg-Salam model and flavor-symmetry breaking

A parity-violating ΔS = 0 effective weak Hamiltonian has been calculated from the Weinberg-Salam model by taking into account QCD corrections and by allowingfor SU(4) flavor symmetry breaking. Non-separable contributions to the parity-violating NNπ amplitude A(n − 0 have been estimated in the framework of the MIT bag model. Separable contributions to this amplitude are mostly due to the operators which mix left and right quark helicities. Double breaking of isospin symmetry, included in the calculation of the separable contributions to hadronic matrix elements (so-called “penguin” diagrams), causes an enhancement in comparison with the “standard” value A(n − 0 = 5 × 10 −8. The Weinberg-Salam model values are likely to be in the region 25 × 10 −8 ⩽ A(n − 0) ⩽ 30 × 10 −8. Experimental consequences are also discussed.