Gross–Neveu model
gross neveu modelThe Gross–Neveu model is a quantum field theory model of Dirac fermions interacting via four fermion interactions in 1 spatial and 1 time dimension It was introduced in 1974 by David Gross and André Neveu as a toy model for quantum chromodynamics, the theory of strong interactions
It consists of N Dirac fermions, ψ1, , ψN The Lagrangian density is
L = ψ ¯ a i ∂ / − m ψ a + g 2 2 N [ ψ ¯ a ψ a ] 2 }=}_\lefti\partial \!\!\!/m\right\psi ^+}}\left^}using the Einstein summation notation where g is the coupling constant If the mass m is nonzero, the model is massive classically, otherwise it enjoys a chiral symmetry
This model has a UN global internal symmetry Note that it does not reduce to the massive Thirring model which is completely integrable
It is a 2dimensional version of the 4dimensional Nambu–JonaLasinio model NJL, which was introduced 14 years earlier as a model of dynamical chiral symmetry breaking but no quark confinement modeled upon the BCS theory of superconductivity The 2dimensional version has the advantage that the 4fermi interaction is renormalizable, which it is not in any higher number of dimensions
Contents
 1 Features of the theory
 2 Generalizations
 3 See also
 4 References
Features of the theory
Gross and Neveu studied this model in the large N limit, expanding the relevant parameters in a 1/N expansion After demonstrating that this and related models are asymptotically free, they found that, in the subleading order, for small fermion masses the bifermion condensate ψ ¯ a ψ a }_\psi ^} acquires a vacuum expectation value VEV and as a result the fundamental fermions become massive They find that the mass is not analytic in the coupling constant g The vacuum expectation value spontaneously breaks the chiral symmetry of the theory
More precisely, expanding about the vacuum with no vacuum expectation value for the bilinear condensate they found a tachyon To do this they solve the renormalization group equations for the propagator of the bifermion field, using the fact that the only renormalization of the coupling constant comes from the wave function renormalization of the composite field They then calculated, at leading order in a 1/N expansion but to all orders in the coupling constant, the dependence of the potential energy on the condensate using the effective action techniques introduced the previous year by Sidney Coleman at the Erice International Summer School of Physics They found that this potential is minimized at a nonzero value of the condensate, indicating that this is the true value of the condensate Expanding the theory about the new vacuum, the tachyon was found to be no longer present and in fact, like the BCS theory of superconductivity, there is a mass gap
They then made a number of general arguments about dynamical mass generation in quantum field theories For example, they demonstrated that not all masses may be dynamically generated in theories which are infraredstable, using this to argue that, at least to leading order in 1/N, the 4dimensional ϕ 4 } theory does not exist They also argued that in asymptotically free theories the dynamically generated masses never depend analytically on the coupling constants
Generalizations
Gross and Neveu considered several generalizations First, they considered a Lagrangian with one extra quartic interaction
L = ψ ¯ a i ∂ / − m ψ a + g 2 2 N [ ψ ¯ a ψ a ] 2 − [ ψ ¯ a γ 5 ψ a ] 2 }=}_\lefti\partial \!\!\!/m\right\psi ^+}}\left^\left^}chosen so that the discrete chiral symmetry ψ → γ 5 ψ \psi } of the original model is enhanced to a continuous U1valued chiral symmetry ψ → e i θ γ 5 ψ }\psi } Chiral symmetry breaking occurs as before, caused by the same VEV However, as the spontaneously broken symmetry is now continuous, a massless Goldstone boson appears in the spectrum Although this leads to no problems at the leading order in the 1/N expansion, massless particles in 2dimensional quantum field theories inevitably lead to infrared divergences and so the theory appears to not exist
Two further modifications of the modified theory, which remedy this problem, were then considered In one modification one increases the number of dimensions As a result, the massless field does not lead to divergences In the other modification, the chiral symmetry is gauged As a result, the Golstone boson is eaten by the Higgs mechanism as the photon becomes massive, and so does not lead to any divergences
See also
 Dirac equation
 Nonlinear Dirac equation
 Thirring model
 Nambu–JonaLasinio model
References
 ^ Gross, David J and Neveu, André 1974 "Dynamical symmetry breaking in asymptotically free field theories" Phys Rev D 10 10: 3235–3253 Bibcode:1974PhRvD103235G doi:101103/PhysRevD103235 CS1 maint: Multiple names: authors list link





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