Up-down quark mass difference: Couplings of pions to nucleons or isobars and low-energy nucleon-nucleon scattering

We investigate effects of the up-down quark mass difference on the couplings of pions to nucleons or isobars. The calculated NN and N couplings are used to derive the long-range one-pion-exchange potential and the intermediate-range attraction for the three different cases: p-p, n-p, and n-n. The resultant potentials are then used to determine numerically the low-energy scattering parameters, including both the scattering length a and the effective range r. Our results indicate that a reasonable up-down quark mass difference such as 10 MeV yields a value for |app|-|ann| of about + 0.6 fm, which is sizable from the experimental point of view. Our results also suggest that the large observed value for |anp|-|ann| can be understood primarily as a manifestation of the pion mass difference. [NUCLEAR REACTIONS Application of quark models to nuclear physics; effects of the up-down quark mass difference on couplings of pions to nucleons or isobars; charge dependence and charge symmetry breaking in low energy nucleon-nucleon scattering.]