Nonlinear 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 2 dimensions, nonlinear σ models are nonrenormalizable. 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.
This represents a Ricci flow having 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 and one has a sensible quantum field theory. 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, leading to an infrared fixed point as well, on account of teleparallelism ("geometrostasis").
O(3) Non-linear Sigma Model
One of the most famous examples, of particular interest due to its topological properties, is the O(3) nonlinear sigma model in 1 + 1 dimensions, with the Lagrangian density
where with the constraint and . 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 we may identify the points at infinity as a single point, i.e. that space-time can be identified with a Riemann Sphere. Since the -field lives on a sphere as well, we have a mapping , the solutions of which are classified by the Second Homotopy group of a 2-sphere. These solutions are called the O(3) Instantons.
- 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
- 'Nonlinear Sigma model' on Scholarpedia