nLab
sigma-model

Context

Quantum field theory

Phyiscs

physics, mathematical physics, philosophy of physics

Surveys, textbooks and lecture notes


theory (physics), model (physics)

experiment, measurement, computable physics

\infty-Chern-Simons theory

∞-Chern-Weil theory

∞-Chern-Simons theory

∞-Wess-Zumino-Witten theory

Ingredients

Definition

Examples

Contents

Idea

A σ\sigma-model is a particular kind of physical theory of certain fields. The basic data describing a specific σ\sigma-model is some kind of “spaceXX, in a category of “spaces” which includes smooth manifolds. We call XX the target space, and we define the “configuration space of fieldsConf ΣConf_\Sigma over a manifold Σ\Sigma to be the mapping space/mapping stack Map(Σ,X)Map(\Sigma, X). That is, a “configuration of fields” over a manifold Σ\Sigma is like an an XX-valued function on Σ\Sigma.

We assign a dimension nn \in \mathbb{N} to our σ\sigma-model, take dimΣndim \Sigma \leq n and assume that target space XX is equipped with a “circle n-bundle with connection”.

For n=1n = 1 this is an ordinary circle bundle with connection and models a configuration of the electromagnetic field on XX. To distinguish this “field” on XX from the fields on Σ\Sigma we speak of a background gauge field. (This remains fixed background data unless and until we pass to second quantization.) A field configuration ΣX\Sigma \to X on Σ\Sigma models a trajectory of a charged particle subject to the forces exerted by this background field.

For n=2n = 2, a circle nn-bundle with connection is a circle 2-group principal 2-bundle or equivalently a bundle gerbe with connection. This models a “higher electromagnetic field”, called a Kalb-Ramond field. Now Σ\Sigma is taken to be 2-dimensional and a map ΣX\Sigma \to X models the trajectory of a string on XX, subject to forces exerted on it by this higher order field.

This pattern continues. In the next dimension a membrane with 3-dimensional worldvolume is charged under a circle 3-bundle with connection, for instance something called the supergravity C-field.

While one can speak of higher bundles in full generality and full analogy to ordinary principal bundles, it is useful to observe that any circle nn-bundle is characterized by a classifying map α:XB nU(1)\alpha : X \to \mathbf{B}^n U(1) in our category of spaces, so we can just think about classifying maps instead. Here U(1)U(1) is the circle group, and B n\mathbf{B}^n denotes its nnth delooping ; thus such a map is also a sort of cocycle in “smooth nnth cohomology of XX with coefficients in U(1)U(1)”. The additional data of a connection refines this to a cocycle in the differential cohomology of XX.

Such connection data \nabla on a circle nn-bundle defines – and is defined by – a notion of higher parallel transport over nn-dimensional trajectories: for closed nn-dimensional Σ\Sigma it defines a map hol:(γ:ΣX)exp(i Σγ *)U(1)hol : (\gamma : \Sigma \to X) \mapsto \exp(i \int_\Sigma \gamma^*\nabla) \in U(1) that sends trajectories to elements in U(1)U(1): the holonomy of \nabla over Σ\Sigma, given by integration of local data over Σ\Sigma. The local data being integrated is called the Lagrangian of the σ\sigma-model. Its integral is called the action functional.

In the quantum σ\sigma-model one considers in turn the integral of the action functional over all of configuration space: the “path integral”. In the classical σ\sigma-model one considers only the critical locus of the action functional (where the rough idea is that the path integral to some approximation localizes around the critical locus). Points in this critical locus are said to be configurations that satisfy the “Euler-Lagrange equations of motion”. These are supposed to be the physically realized trajectories among all of them, in the classical approximation.

Finally, just like an ordinary circle group-principal bundle has an associated vector bundle once we fix a representation of U(1)U(1) to be the fibers, any “circle n-bundle” has an associated “n-vector bundle” once we fix a “∞-representationρ:B nU(1)nVect\rho : \mathbf{B}^n U(1) \to n Vect on “n-vector spaces”. Just as for the ordinary U(1)U(1), here we usually pick the canonical 1-dimensional such “representation”. Finally, we define bundles V Σ:Conf Σ𝒞V_\Sigma : Conf_\Sigma \to \mathcal{C} of “internal states” by transgression of these associated bundles.

The passage from principal ∞-bundles to associated ∞-bundles is necessary for the description of the quantum σ\sigma-model: it assigns in positive codimension spaces of sections of these associated bundles. For a 1-categorical description of the resulting QFT ordinary vector bundles (assigned in codimension 1) would suffice, but the σ\sigma-model should determine much more: an extended quantum field theory. This requires sections of higher vector bundles. For instance for n=2n = 2 some boundary conditions of the σ\sigma-model are given by sections of the background 2-vector bundle: these are the twisted vector bundles known as the Chan-Paton bundles on the boundary-D-branes of the string. (…)

We now try to fill this with life by spelling out some standard examples. Further below we look at precise formalizations of the situation.

Terminology and history

In physics one tends to speak of a model if one specifies a particular quantum field theory for describing a particular situation, for instance by specifying a Lagrangian or local action functional on some configuration space. This is traditionally not meant in the mathematical sense of model of some theory. But in light of progress of mathematically formalizing quantum field theory (see FQFT and AQFT), it can with hindsight be interpreted in this way:

a σ\sigma-model is supposed to be a type of model for the theory called quantum field theory. This sounds like a tautology, but much effort in mathematical physics is devoted to eventually making this a precise statement. In special cases and toy examples this has been achieved, but for the examples that seem to be directly relevant for the phenomenological description of the observed world, lots of clues still seem to be missing.

As to the ”σ\sigma” in ”σ\sigma-model”: back in the 1960s people were interested in a hypothetical particle called the σ\sigma-particle. Murray Gell-Mann came up with a theory of them. It was called ‘the σ\sigma-model’. It was an old-fashioned field theory where the field took values in a vector space. Then someone came up with a modified version of the σ-model where the field took values in some other manifold and this was called ‘the nonlinear σ-model’.

While the parameter space (the domain space of the fields) of the original σ\sigma-models was supposed to be our spacetime and the target space was some abstract space, with the advent of string theory the nonlinear σ\sigma-models gained importance as quantum field theories whose target space is spacetime XX and whose parameter space is some low dimensional space, usually denoted Σ\Sigma. A field configuration ΣX\Sigma \to X is then interpreted as being the trajectory of an extended fundamental particle – a fundamental brane – in XX, and the σ\sigma-model describes the quantum mechanics of that brane propagating in XX.

In particular the quantum mechanics of a relativistic particle propagating on XX is described by a σ\sigma-model on the real line Σ=\Sigma = \mathbb{R} – the worldline of the particle.

In string theory one considers 2-dimensional Σ\Sigma and thinks of maps ΣX\Sigma \to X as being the worldsheets of the trajectory of a string propagating in spacetime.

In the context of 11-dimensional supergravity there is a σ\sigma-model with 3-dimensional Σ\Sigma, describing the propagation of a membrane in spacetime.

Exposition of classical sigma-models

We survey, starting from the very basics, classical field theory aspects of σ\sigma-models that describe dynamics of particles, strings and branes on geometric target spaces.

The content of this section is at

Exposition of higher gauge theories as σ\sigma-models

We discuss how gauge theories and their higher analogs are naturally regarded as σ\sigma-models.

The content of this section is at

Exposition of quantum σ\sigma-models

Above we have discussed some standard classical sigma-models and higher gauge theories as sigma-models, also mostly classically. Here we talk about the quantization of these models (or some of them) to QFTs: quantum σ\sigma-models .

The content of this section is at

See there for discussion of string topology, Gromov-Witten theory, Chern-Simons theory.

Exposition of a general abstract formulation

We give a leisurely exposition of a general abstract formulation σ\sigma-models, aimed at readers with a background in category theory but trying to assume no other prerequisites.

What is called an nn-dimensional σ\sigma-model is first of all an instance of an nn-dimensional quantum field theory (to be explained). The distinctive feature of those quantum field theories that are σ\sigma-models is that

  1. these arise from a simpler kind of field theory – called a classical field theory – by a process called quantization

  2. moreover, this simpler kind of field theory encoded bygeometric data in a nice way: it describes physical configuration spaces that are mapping spaces into a geometric space equipped with some differential geometric structure.

We give expositions of these items step-by-step:

  1. Quantum field theory

  2. Classical field theory

  3. Quantization

  4. Classical sigma-models

  5. Quantum sigma-models

We draw from (FHLT, section 3).

The content of this section is at

Exposition of second quantization of σ\sigma-models

We discuss second quantization in the context of σ\sigma-models.

The content of this section is at

Examples

Non-topological σ\sigma-models

Topological σ\sigma-models

References

A standard reference on 2-dimensional string σ\sigma-models is

Further discussion of sigma-models in the context of string theory is for instance in

  • C. Callan, L. Thorlacius, Sigma models and string theory, Particles, Strings and Supernovae, Volumes I and II. Proceedings of the Theoretical Advanced Study Institute in Elementary Particle Physics, held June 6 - July 2, 1988, at Brown University, Providence, Rhode Island. Edited by A. Jevicki and C.-I. Tan. Published by World Scientific, New York, 1989, p.795 (pdf)

  • Arkady Tseytlin, Sigma model approach to string theory effective actions with tachyons, J. Math.Phys.42:2854-2871 (2001) (arXiv:hep-th/0011033)

First indications on how to formalize σ\sigma-models in a higher categorical context were given in

A grand picture developing this approach further is sketched in

A discussion of 2- or (2+1)-dimensional Σ\Sigma-models whose target is an derived stack/infinity-stack is in

More discussion of the latter is at geometric infinity-function theory.

A discussion of σ\sigma-models of higher gauge theory type is at

Concrete applications of σ\sigma-models with target stacks (typically smooth ones, hence smooth groupoids) in string theory and supergravity are discussed for instance in

Revised on January 23, 2014 03:28:19 by David Corfield (87.112.108.67)