Closed-channel parameters of Feshbach resonances

TL;DR

This paper explores how the properties of the closed channel in Feshbach resonances are characterized by experimental observables, revealing insensitivity in two-body data for deep potentials and proposing a new measurement method for the closed-channel scattering length.

Contribution

It demonstrates the insensitivity of two-body observables to closed channel properties in deep potential Feshbach resonances and introduces a photoassociation experiment to measure the closed-channel scattering length.

Findings

Two-body observables are insensitive to closed channel properties in deep potentials.

Short-range correlations and three-body observables depend on the closed-channel scattering length.

A proposed photoassociation experiment can measure the closed-channel scattering length.

Abstract

This work investigates how the closed channel of a Feshbach resonance is characterised by experimental observables. Surprisingly, it is found that the two-body observables associated with the Feshbach resonance can be insensitive to the properties of the closed channel. In particular, it is impossible in this situation to determine the energy of the bound state causing the resonance from the usual experimental data. This is the case for all magnetic Feshbach resonances in ultracold atoms, due to their deep two-body interaction potentials. This insensitivity highlights a major difference with Feshbach resonances that involve shallow interaction potentials, such as hadron resonances. It appears however that short-range two-body correlations and three-body observables are affected by a parameter of the closed channel called the "closed-channel scattering length". A photoassociation experiment is proposed to measure this parameter in ultracold atom systems.