# Modeling atom-atom interactions at low energy by Jost-Kohn potentials

## TL;DR

This paper revisits Jost and Kohn's methods to model low-energy atom-atom interactions using short-range potentials, demonstrating their ability to account for resonances and reproduce contact interaction results.

## Contribution

It extends the Jost-Kohn potential framework to model resonant finite-range interactions, including zero-energy resonances, and analyzes their impact on scattering parameters.

## Key findings

- Jost-Kohn potentials can model zero-energy resonances.
- The phase shift depends analytically on the bound state energy.
- Effective range can become negative for small binding energies.

## Abstract

More than 65 years ago, Jost and Kohn [R. Jost and W. Kohn, {Phys. Rev.} {\bf 87}, 977 (1952)] derived an explicit expression for a class of short-range model potentials from a given effective range expansion with the $s$-wave scattering length $a_s$ being negative. For $a_s >0$, they calculated another class of short-range model potentials [R. Jost and W. Kohn, { Dan. Mat. Fys. Medd} {\bf 27}, 1 (1953)] using a method based on an adaptation from Gelfand-Levitan theory [I. M. Gel'fand and B. M. Levitan, { Dokl. Akad. Nauk. USSR} {\bf 77}, 557-560 (1951)] of inverse scattering. We here revisit the methods of Jost and Kohn in order to explore the possibility of modeling resonant finite-range interactions at low energy. We show that the Jost-Kohn potentials can account for zero-energy resonances. The $s$-wave phase shift for positive scattering length is expressed in an analytical form as a function of the binding energy of a bound state. We show that, for small binding energy, both the scattering length and the effective range are strongly influenced by the binding energy; and below a critical binding energy the effective range becomes negative provided the scattering length is large. As a consistency check, we carry out some simple calculations to show that Jost-Kohn potentials can reproduce the standard results of contact interaction in the limit of the effective range going to zero.