Non-linear sigma model
In quantum field theory, a nonlinear σ model describes a scalar field Σ which takes on values in a nonlinear manifold called the target manifold T. The non-linear σ-model was introduced by Gell-Mann & Lévy (1960, section 6), who named it after a field corresponding to a spin 0 meson called σ in their model. 
In the coordinate notation, with the coordinates Σa, a = 1, ..., n where n is the dimension of T,
In more than two dimensions, nonlinear σ models contain a dimensionful coupling constant and are thus not perturbatively renormalizable. Nevertheless, they exhibit a non-trivial ultraviolet fixed point of the renormalization group both in the lattice formulation and in the double expansion originally proposed by Kenneth G. Wilson.
In both approaches, the non-trivial renormalization-group fixed point found for the O(n)-symmetric model is seen to simply describe, in dimensions greater than two, the critical point separating the ordered from the disordered phase. In addition, the improved lattice or quantum field theory predictions can then be compared to laboratory experiments on critical phenomena, since the O(n) model describes physical Heisenberg ferromagnets and related systems. The above results point therefore to a failure of naive perturbation theory in describing correctly the physical behavior of the O(n)-symmetric model above two dimensions, and to the need for more sophisticated non-perturbative methods such as the lattice formulation.
This means they can only arise as effective field theories. New physics is needed at around the distance scale where the two point connected correlation function is of the same order as the curvature of the target manifold. This is called the UV completion of the theory. There is a special class of nonlinear σ models with the internal symmetry group G *. If G is a Lie group and H is a Lie subgroup, then the quotient space G/H is a manifold (subject to certain technical restrictions like H being a closed subset) and is also a homogeneous space of G or in other words, a nonlinear realization of G. In many cases, G/H can be equipped with a Riemannian metric which is G-invariant. This is always the case, for example, if G is compact. A nonlinear σ model with G/H as the target manifold with a G-invariant Riemannian metric and a zero potential is called a quotient space (or coset space) nonlinear σ model.
This model proved to be relevant in string theory where the two-dimensional manifold is named worldsheet. Proof of renormalizability was given by Daniel Friedan. He showed that the theory admits a renormalization group equation, at the leading order of perturbation theory, in the form
being the Ricci tensor of the target manifold.
This represents a Ricci flow, obeying Einstein field equations for the target manifold as a fixed point. The existence of such a fixed point is relevant, as it grants, at this order of perturbation theory, that conformal invariance is not lost due to quantum corrections, so that the quantum field theory of this model is sensible (renormalizable).
Further adding nonlinear interactions representing flavor-chiral anomalies results in the Wess–Zumino–Witten model, which augments the geometry of the flow to include torsion, preserving renormalizability and leading to an infrared fixed point as well, on account of teleparallelism ("geometrostasis").
O(3) non-linear sigma model
A celebrated example, of particular interest due to its topological properties, is the O(3) nonlinear σ-model in 1 + 1 dimensions, with the Lagrangian density
where with the constraint and μ=1,2.
This model allows for topological finite action solutions, as at infinite space-time the Lagrangian density must vanish, meaning at infinity. Therefore, in the class of finite-action solutions, one may identify the points at infinity as a single point, i.e. that space-time can be identified with a Riemann sphere.
- Sigma model
- Chiral model
- Little Higgs
- Skyrmion, a soliton in non-linear sigma models
- WZW model
- Fubini–Study metric, a metric often used with non-linear sigma models.
- Ricci flow
- Scale invariance
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- Gürsey, F. (1960). "On the symmetries of strong and weak interactions". Il Nuovo Cimento 16 (2): 230–240. doi:10.1007/BF02860276.
- Zinn-Justin, Jean (2002). Quantum Field Theory and Critical Phenomena. Oxford University Press.
- Cardy, John L. (1997). Scaling and the Renormalization Group in Statistical Physics. Cambridge University Press.
- Brezin, Eduard; Zinn-Justin, Jean (1976). "Renormalization of the nonlinear sigma model in in 2 + epsilon dimensions". Physical Review Letters 36: 691–693. Bibcode:1976PhRvL..36..691B. doi:10.1103/PhysRevLett.36.691.
- Friedan, D. (1980). "Nonlinear models in 2+ε dimensions". PRL 45 (13): 1057. Bibcode:1980PhRvL..45.1057F. doi:10.1103/PhysRevLett.45.1057.
- Witten, E. (1984). "Non-abelian bosonization in two dimensions". Communications in Mathematical Physics 92 (4): 455–472. Bibcode:1984CMaPh..92..455W. doi:10.1007/BF01215276.
- Braaten, E.; Curtright, T. L.; Zachos, C. K. (1985). "Torsion and geometrostasis in nonlinear sigma models". Nuclear Physics B 260 (3–4): 630. Bibcode:1985NuPhB.260..630B. doi:10.1016/0550-3213(85)90053-7.