The 2+ doublet in 8Be: an example of accidental resonance degeneracy

By means of a numerical analysis of the 2 + doublet in 8 Be, we show that accidental degeneracies of two resonant states mixed by a Hermitian interaction are associated with the occurrence of a simple and a double pole of the S -matrix in the complex energy plane. A true crossing may be brought about by the variation of just two parameters.