# Above threshold scattering about a Feshbach resonance for ultracold atoms in an optical collider

## TL;DR

This paper demonstrates a laser-based collider method to accurately locate a narrow Feshbach resonance in ultracold rubidium atoms above threshold, using collision-induced shifts in outgoing cloud positions.

## Contribution

It introduces a novel approach to characterize Feshbach resonances above threshold by analyzing collision shifts, extending beyond traditional near-zero energy measurements.

## Key findings

- Resonance position determined accurately above threshold.
- Collision shifts cross zero at the resonance.
- Results agree well with theoretical predictions.

## Abstract

Ultracold atomic gases have realised numerous paradigms of condensed matter physics where control over interactions has crucially been afforded by tunable Feshbach resonances. So far, the characterisation of these Feshbach resonances has almost exclusively relied on experiments in the threshold regime near zero energy. Here we use a laser-based collider to probe a narrow magnetic Feshbach resonance of rubidium above threshold. By measuring the overall atomic loss from colliding clouds as a function of magnetic field, we track the energy-dependent resonance position. At higher energy, our collider scheme broadens the loss feature, making the identification of the narrow resonance challenging. However, we observe that the collisions give rise to shifts in the centre-of-mass positions of outgoing clouds. The shifts cross zero at the resonance and this allows us to accurately determine its location well above threshold. Our inferred resonance positions are in excellent agreement with theory.