Compositeness of near-threshold $s$-wave resonances

Abstract

The near-threshold clustering phenomenon is well understood by the low-energy universality, for shallow bound states below the threshold. Nevertheless, the characteristics of resonances slightly above the threshold still lack thorough elucidation. We introduce a novel probabilistic interpretation scheme for complex compositeness of resonances, in which resonances with large decay widths fall outside the domain where their internal structure can be interpreted probabilistically. Employing this scheme to analyze resonances via the effective range expansion, we demonstrate that near-threshold resonances have a small composite fraction, in sharp contrast to shallow bound states below the threshold.