Charge asymmetry and charge dependence of two-nucleon scattering

The electromagnetic (EM) contributions to nucleon-nucleon scattering are subtracted from the experimental scattering length in the 1 S 0 partial wave. The resulting nuclear proton-proton scattering length depends strongly on the unknown nuclear potential at small distances. The comparison between proton-proton and neutron-neutron data is therefore ambiguous. The amount of nuclear charge asymmetry in the 1 S 0 scattering length cannot be established. In contrast, the difference between the experimental proton-neutron and neutron-neutron 1 S 0 scattering lengths yields a quantitative estimate for the charge dependence of the nucleon-nucleon interaction, not invalidated by the subtraction of EM effects. A constraint on the parametrisation of the nuclear potential at small relative distances arising from nonrelativistic quantum mechanics is given.