Low-energy parameters of neutron-neutron interaction in the effective-range approximation

The effect of the mass difference between the charged and neutral pions on the low-energy parameters of nucleon-nucleon interaction in the 1 S 0 state is studied in the effective-range approximation. On the basis of experimental values of the singlet parameters of neutron-proton scattering and the experimental value of the virtual-state energy for the neutron-neutron systemin the 1 S 0 state, the following values were obtained for the neutron-neutron scattering length and effective range: a nn = −16.59(117) fm and r nn = 2.83(11) fm. The calculated values agree well with present-day experimental results.