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Feshbach resonance
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<p>In <a href="/facts/Physics/a7aH2y08">physics</a>, a Feshbach resonance can occur upon collision of two slow <a href="/facts/Atom/FoQ4kGN5">atoms</a>, when they temporarily stick together forming an unstable compound with short lifetime (so-called resonance). It is a feature of <a href="/facts/Many-body/n2rIxgwv">many-body</a> systems in which a <a href="/facts/Bound_state/NndW5JQl">bound state</a> is achieved if the coupling(s) between at least one internal <a href="/facts/Degrees_of_freedom_(physics_and_chemistry)/Rrjl0lnq">degree of freedom</a> and the <a href="/facts/Reaction_coordinate/3ttAGUY0">reaction coordinates</a>, which lead to <a href="/facts/Dissociation_(chemistry)/gDRGdeSN">dissociation</a>, vanish. The opposite situation, when a bound state is not formed, is a <a href="/facts/Shape_resonance/2pbGHT50">shape resonance</a>. It is named after <a href="/facts/Herman_Feshbach/0TKoFTNw">Herman Feshbach</a>, a physicist at <a href="/facts/MIT/Xk49vNA8">MIT</a>. </p><p>Feshbach resonances have become important in the study of <a href="/facts/Ultracold_atom/LdM0IeD7">cold atoms</a> systems, including <a href="/facts/Fermi_gas/D00lFTqW">Fermi gases</a> and <a href="/facts/Bose%E2%80%93Einstein_condensate/76peCwfW">Bose–Einstein condensates</a> (BECs). In the context of <a href="/facts/Scattering/nMou7lEH">scattering</a> processes in many-body systems, the Feshbach resonance occurs when the energy of a bound state of an <a href="/facts/Interatomic_potential/EgJjsSeh">interatomic potential</a> is equal to the <a href="/facts/Kinetic_energy/aJRxUd4p">kinetic energy</a> of a colliding pair of atoms. In experimental settings, the Feshbach resonances provide a way to vary interaction strength between atoms in the cloud by changing scattering length, asc, of elastic collisions. For atomic species that possess these resonances (like K39 and K40), it is possible to vary the interaction strength by applying a uniform magnetic field. Among many uses, this tool has served to explore the transition from a <a href="/facts/Bose%E2%80%93Einstein_condensate/76peCwfW">BEC</a> of fermionic molecules to weakly interacting fermion-pairs the <a href="/facts/BCS_theory/KmlyhxPS">BCS</a> in Fermi clouds. For the BECs, Feshbach resonances have been used to study a spectrum of systems from the non-interacting ideal Bose gases to the unitary regime of interactions. </p>