Compositeness of near-threshold eigenstates with Coulomb plus short-range interactions

TL;DR

This paper develops a formalism to analyze the internal structure of near-threshold eigenstates with Coulomb plus short-range interactions, revealing how Coulomb strength affects compositeness and threshold behavior.

Contribution

It derives a new weak-binding relation incorporating Coulomb effects and studies how Coulomb interaction strength influences the compositeness of near-threshold states.

Findings

Coulomb interaction qualitatively alters pole trajectories and internal structure.

Strong Coulomb interaction suppresses compositeness enhancement near threshold.

Weak Coulomb interaction allows near-threshold states to remain predominantly composite.

Abstract

We investigate the internal structure of near-threshold $s$-wave eigenstates in a two-body system with Coulomb plus short-range interactions. Using a nonrelativistic effective field theory, we derive the expression for the compositeness in terms of the energy derivative of the self-energy, which is applicable to the present system with the non-separable Coulomb interaction. For near-threshold states, the compositeness can be written solely in terms of the Coulomb scattering length, the Coulomb effective range, and the Bohr radius, providing the weak-binding relation in the presence of the Coulomb interaction. We numerically study the pole trajectories and the compositeness and find that the Coulomb interaction qualitatively modifies the threshold behavior of the poles and the internal structure of the eigenstates. We show that when the Coulomb interaction is relatively strong, the enhancement of the compositeness near the threshold is absent, in contrast to purely short-range interactions. On the other hand, for a weak Coulomb interaction, a remnant of short-range universality survives, and near-threshold bound states tend to be composite dominant. Furthermore, even resonances are dominated by the composite component in the presence of the Coulomb interaction, owing to their continuous connection to the bound-state regime. We apply the formalism to realistic systems with near-threshold eigenstates, including exotic hadrons and nuclei.