A Feshbach resonance is a resonance of a many-body system in which a bound state is achieved if the coupling(s) between an internal degree(s) of freedom and the reaction coordinates which lead to dissociation vanish. The opposite situation, when a bound state is not formed, is a shape resonance.
Feshbach resonances have become important in the study of fermi gases, as these resonances allow for the creation of Bose Einstein Condensates (BECs). In the context of a BEC, the Feshbach resonance occurs when the energy of a bound state of an interatomic potential is equal to the kinetic energy of a colliding pair of atoms, which have hyperfine structure coupled via Coulomb or exchange interactions. This condition is rare, but can be satisfied in ultracold alkali atoms.
The Feshbach resonance is named after Herman Feshbach.
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