Alpha-Particle Resonance

A phase-shift analysis of the elastic $p-t$ cross section in the energy range 0.1 to 0.76 MeV has been made. It is shown that the low peak observed at 0.3 MeV is consistent with a $0^{+}$ resonance at 0.5±0.1 MeV. The shape of the observed cusp in the $p-t$ elastic cross section at the threshold energy of the $n$-${\mathrm{He}}^3$ channel is used to demonstrate that the energy behavior of the singlet and triplet $s$-wave phase obtained must be qualitatively unique. A direct comparison is made of the Breit-Wigner single-level approximation and the alternative two-channel scattering length description of the state.