  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.
