Feshbach resonance

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In the field of physics, a Feshbach resonance, named after Herman Feshbach, is a feature of many-body systems in which a bound state is achieved if the coupling(s) between at least one internal degree 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 in our local conditions, but can be satisfied in ultracold alkali atoms.


Consider a general quantum scattering event between two particles. In this reaction, there are two reactant particles denoted by A and B, and two product particles denoted by A' and B' . For the case of a reaction (such a as nuclear reaction), we may denote this scattering event by

A + B \rightarrow A' + B' or A(B,B')A'.

The combination of the species and quantum states of the two reactant particles before or after the scattering event is referred to as a reaction channel. Specifically, the species and states of A and B constitute the entrance channel, while the types and states of A' and B' constitute the exit channel. An energetically accessible reaction channel is referred to as an open channel, whereas a reaction channel forbidden by energy conservation is referred to as a closed channel.

Consider the interaction of two particles A and B in an entrance channel C. The positions of these two particles are given by \vec{r}_A and \vec{r}_B, respectively. The interaction energy of the two particles will usually depend only on the magnitude of the separation R \equiv |\vec{r}_A - \vec{r}_B|, and this function, sometimes referred to as a potential energy curve, is denoted by V_c(R). Often, this potential will have a pronounced minimum and thus admit bound states.

The total energy of the two particles in the entrance channel is

 E = T + V_C(R) + \Delta(\vec{P}),

where  T denotes the total kinetic energy of the relative motion (center-of-mass motion plays no role in the two-body interaction),  \Delta is the contribution to the energy from couplings to external fields, and \vec{P} represents a vector of one or more parameters such as magnetic field or electric field. We consider now a second reaction channel, denoted by D, which is closed for large values of R. Let this potential curve  V_D(R) admit a bound state with energy  E_D. .

A Feshbach resonance occurs when

 E_D \approx T + V_C(R) + \Delta(\vec{P}_0)

for some range of parameter vectors \lbrace\vec{P}_0\rbrace. When this condition is met, then any coupling between channel C and channel D can give rise to significant mixing between the two channels; this manifests itself as a drastic dependence of the outcome of the scattering event on the parameter or parameters that control the energy of the entrance channel.