Virtual state of the three nucleon system

The existence of a virtual state of the three nucleon system is established on the basis of three different analyses. Values for its pole position and residue in the doublet, $s$-wave, $n-d$ elastic scattering amplitude, are obtained from a fit to the experimental data, from partial wave dispersion relations, and from an exact three-particle, separable potential calculation. The calculations indicate that these parameters are determined mainly by the one-nucleon exchange mechanism and the doublet scattering length $a_2$. For $a_2=0.65\mathrm{}$ fm our best calculation gives an energy of 0.482 MeV below the elastic threshold, on the second Riemann sheet, and a residue parameter ${C_{\nu }}^2=0.0504\mathrm{}$, where ${C_{\nu }}^2$ is defined in analogy to the triton asymptotic normalization parameter.