Gamow shell model description of exceptional point in ${}^7$Li

Abstract

We report the first identification of an exceptional point (EP) within the Gamow Shell Model in the Coupled Channels representation (GSM-CC). In the spectrum of ${}^7$Li, an EP is found for the $5/2^{-}$ doublet, where the two states coalesce in both energy and width, the phase rigidity vanishes, and the S-matrix develops a double pole. These features manifest directly in observables: the elastic cross section acquires a split-peak structure, and the phase shift shows a single $2\ddot{I}€$ jump. This work demonstrates that GSM-CC provides a powerful framework to explore EP phenomena in nuclei and their experimental signatures.