The S-matrix pole structure of the ${\mathrm{J}}^{\mathrm{\pi }}$${=(3/2\mathrm{}}^{+}$ resonance in ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ is obtained from R-matrix parameters for the system. Two poles are found on different unphysical Riemann sheets. One is a conventional resonance that is primarily responsible for structure in the n-α total cross section. The other is a ‘‘shadow’’ pole that contributes to the large values of the cross section for the reaction ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(d,n${)\mathrm{}}^4$He at low energies. It is the first experimental evidence for the existence of shadow poles in nuclear and particle physics. The two-pole structure of the resonance accounts for different peak positions and widths in the amplitudes for the n-α total cross section and for the ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(d,n) reaction cross section.

• Received 11 May 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.763