Compositeness of near-threshold s-wave resonances

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 the resonances with unphysically large decay widths are inherently excluded. Employing this scheme to analyze resonances via the effective range expansion, we demonstrate that near-threshold resonances have small composite fraction, in sharp contrast to shallow bound states below the threshold.