Four-fermion interactions
- Four-fermion interactions
In quantum field theory, fermions are described by anticommuting spinor fields. A four-fermi interaction describes a local interaction between four fermionic fields at a point. Local here means that it all happens at the same spacetime point. This might be an effective field theory or it might be fundamental.
Some examples are the following:
*Fermi's theory of the weak interaction. The interaction term has a V-A (vector minus axial) form.
*The Gross-Neveu model. This is a four-fermi theory of Dirac fermions without chiral symmetry and as such, it may or may not be massive.
*The Thirring model. This is a four-fermi theory of fermions with a vector coupling.
*The Nambu-Jona-Lasinio model. This is a four-fermi theory of Dirac fermions with chiral symmetry and as such, it has no bare mass.
A nonrelativistic example is the BCS theory at large length scales with the phonons integrated out so that the force between two dressed electrons is approximated by a contact term.
In four space-time dimensions, such theories are not renormalisable.
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