Pole structure of the J pi =3/2(+) resonance in 5He.
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The S-matrix pole structure of the Jπ=(3/2+ resonance in He5 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 H3(d,n)4He 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 H3(d,n) reaction cross section.