Mass-difference effects in the NN system

It is shown that the mass-difference effects of the difference between the pp and nn scattering lengths can become anomalously large by off-shell effects. The result shows clearly how this mass-difference correction depends on the off-shell behaviour of an interaction model. A comparison of theoretical results and experimental data gives a constraint on the half-shell function at the threshold. Charge-symmetry-breaking effects play an important role in the description of the data by realistic NN potentials such as the Paris potential. (author)