Quantum field theoretic model of metastable resonant spin-singlet state   of the np pair
  The np pair in the spin-singlet state is treated as a Cooper np-pair within the extension of the Nambu-Jona-Lasinio model of light nuclei, describing the deuteron as a Cooper np-pair in the spin-triplet state. For the Cooper np-pair in the spin-singlet state we compute the binding energy and express the S-wave scattering length of np scattering in the spin-singlet state in terms of the binding energy. The theoretical value of the S-wave scattering length of np scattering agrees well with the experimental data. 
A.N.Ivanov, M.Cargnelli, M.Faber, H.Fuhrmann, V.A.Ivanova, J.Marton,   N.I.Troitskaya, J.Zmeskal