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Wess-Zumino-Witten model

Context

\infty-Wess-Zumino-Witten theory

Quantum field theory

Physics

physics, mathematical physics, philosophy of physics

Surveys, textbooks and lecture notes


theory (physics), model (physics)

experiment, measurement, computable physics

Contents

Idea

The Wess-Zumino-Witten model (or WZW model for short, also called Wess-Zumino-Novikov-Witten model, or short WZNW model) is a 2-dimensional sigma-model quantum field theory whose target space is a Lie group.

This may be regarded as the boundary theory of Chern-Simons theory for Lie group GG.

The vertex operator algebras corresponding to the WZW model are current algebras.

Action functional

For GG a Lie group, the configuration space of the WZW over a 2-dimensional manifold Σ\Sigma is the space of smooth functions g:ΣGg : \Sigma \to G.

The action functional of the WZW sigma-model is the sum of two terms, a kinetic term and a topological term

S WZW=S kin+S top. S_{WZW} = S_{kin} + S_{top} \,.

Kinetic term

The Lie group canonically carries a Riemannian metric and the kinetic term is the standard one for sigma-models on Riemannian target spaces.

Topological term – WZW term

Let GG be compact and simply connected.

Then by infinity-Chern-Weil theory the Killing form invariant polynomial on the Lie algebra 𝔤\mathfrak{g} induces a circle 3-bundle with connection on the smooth moduli stack BG conn\mathbf{B}G_{conn} of GG-principal bundles with connection.

CS c:BG connB 3U(1) conn. CS_{\mathbf{c}} : \mathbf{B}G_{conn} \to \mathbf{B}^3 U(1)_{conn} \,.

This is the Lagrangian for GG-Chern-Simons theory. By looping this, including a differential twist, there is induced canonically a circle 2-bundle with connection

WZW c:GB 2U(1) conn. WZW_{\mathbf{c}} : G \to \mathbf{B}^2 U(1)_{conn} \,.

The construction of the canonical morphism WZW cWZW_{\mathbf{c}} goes as follows. Consider, inside BG conn\mathbf{B}G_{conn} the substack BG dR\mathbf{B}G_{\flat dR} of trivialized principal GG-bundles with flat connections. The morphism CS cCS_{\mathbf{c}}, restricted to BG dR\mathbf{B}G_{\flat dR}, factors as

BG dRΩ 3()B 3U(1) conn \mathbf{B}G_{\flat dR}\to \Omega^3(-) \to \mathbf{B}^3 U(1)_{conn}

where Ω 3()\Omega^3(-) is identified with the 3-stack of trivialized circle bundles with connection whose connection form lives entirely in degree 3. Since BG dRBG conn\mathbf{B}G_{\flat dR}\to \mathbf{B}G_{conn} factors through the stack BG\flat\mathbf{B}G of principal GG-bundles with flat conenctions, we have a homotopy commutative diagram

BG dR CS c Ω 3() BG B 3U(1) conn \array{ \mathbf{B}G_{\flat dR}&\to^{CS_{\mathbf{c}}}& \Omega^3(-)\\ \downarrow && \downarrow\\ \flat\mathbf{B}G& \to & \mathbf{B}^3 U(1)_{conn} }

and so a canonically induced morphism WZW cWZW_{\mathbf{c}} between the homotopy fibers over the distinguished points. Since we have homotopy pullbacks

G BG dR * BG \array{ G&\to& \mathbf{B}G_{\flat dR}\\ \downarrow && \downarrow\\ {*}& \to & \flat\mathbf{B}G }

and

B 2U(1) conn Ω 3() * B 3U(1) conn, \array{ \mathbf{B}^2 U(1)_{conn}&\to & \Omega^3(-)\\ \downarrow && \downarrow\\ {*}& \to & \mathbf{B}^3 U(1)_{conn} },

the morphism WZW cWZW_{\mathbf{c}} can naturally be seen as a morphism from GG (as a smooth manifold) to the 2-stack B 2U(1) conn\mathbf{B}^2 U(1)_{conn} of circle 2-bundles with connection. In other words, if GG is a compact simply connected simple Lie group, the differential refinement CS cCS_{\mathbf{c}} of the degree 4 characteristic class c\mathbf{c} provided by Chern-Simons theory naturally induces a circle 2-bundle with connection over the smooth manifold underlying the Lie group GG.

The surface holonomy of this is the topological part of the WZW action functional:

exp(iS top(g))=hol Σ(g *WZW c). \exp(i S_{top}(g)) = hol_{\Sigma}( g^* WZW_{\mathbf{c}} ) \,.

Properties

Equations of motion

The variational derivative of the WZW action functional is

δS WZW(g)=k2πi Σ(g 1δg),(g 1¯g). \delta S_{WZW}(g) = -\frac{k}{2 \pi i } \int_\Sigma \langle (g^{-1}\delta g), \partial (g^{-1}\bar \partial g) \rangle \,.

Therefore the classical equations of motion for function g:ΣGg \colon \Sigma \to G are

(g 1¯g)=0¯(gg 1)=0. \partial(g^{-1}\bar \partial g) = 0 \;\;\;\;\;\; \Leftrightarrow \;\;\;\;\;\; \bar \partial(g \partial g^{-1}) = 0 \,.

The space of solutions to these equations is small. However, discussion of the quantization of the theory (below) suggests to consider these equations with the real Lie group GG replaced by its complexification to the complex Lie group G()G({\mathbb{C}}). Then the general solution to the equations of motion above has the form

g(z,z¯)=g (z)g r(z¯) 1 g(z,\bar z) = g_{\ell}(z) g_r(\bar z)^{-1}

where hence g :ΣG()g_{\ell} \colon \Sigma \to G(\mathbb{C}) is any holomorphic function and g rg_r similarly any anti-holomorphic function.

(e.g. Gawedzki 99 (3.18), (3.19))

Holography and Rigorous construction

By the AdS3-CFT2 and CS-WZW correspondence (see there for more details) the 2d WZW CFT on GG is related to GG-Chern-Simons theory in 3d3d.

In fact a rigorous constructions of the GG-WZW model as a rational 2d CFT is via the FRS-theorem on rational 2d CFT, which constructs the model as a boundary field theory of the GG-Chern-Simons theory as a 3d TQFT incarnated via a Reshetikhin-Turaev construction.

D-branes for the WZW model

The characterization of D-brane submanifolds for the open string WZW model on a Lie group GG comes from two consistency conditions:

  1. geometrical condition:

    For the open string CFT to still have current algebra worldsheet symmetry, hence for half the current algebra symmetry of the closed WZW string to be preserved, the D-brane submanifolds need to be conjugacy classes of the group manifold (see e.g. Alekseev-Schomerus for a brief review and further pointers). These conjugacy classes are therefore also called the symmetric D-branes.

    Notice that these conjugacy classes are equivalently the leaves of the foliation induced by the canonical Cartan-Dirac structure on GG, hence (by the discussion at Dirac structure), the leaves induced by the Lagrangian sub-Lie 2-algebroids of the Courant Lie 2-algebroid which is the higher gauge groupoid (see there) of the background B-field on GG.(It has been suggested by Chris Rogers that such a foliation be thought of as a higher real polarization.)

  2. cohomological condition:

    In order for the Kapustin-part of the Freed-Witten-Kapustin anomaly of the worldsheet action functional of the open WZW string to vanish, the D-brane must be equipped with a Chan-Paton gauge field, hence a twisted unitary bundle (bundle gerbe module) of some rank nn \in \mathbb{N} for the restriction of the ambient B-field to the brane.

    For simply connected Lie groups only the rank-1 Chan-Paton gauge fields and their direct sums play a role, and their existence corresponds to a trivialization of the underlying BU(1)\mathbf{B}U(1)-principal 2-bundle (U(1)U(1)-bundle gerbe) of the restriction of the B-field to the brane. There is then a discrete finite collection of symmetric D-branes = conjugacy classes satisfying this condition, and these are called the integral symmetric D-branes. (Alekseev-Schomerus, Gawedzki-Reis). As observed in Alekseev-Schomerus, this may be thought of as identifying a D-brane as a variant kind of a Bohr-Sommerfeld leaf.

    More generally, on non-simply connected group manifolds there are nontrivial higher rank twisted unitary bundles/Chan-Paton gauge fields over conjugacy classes and hence the cohomological “integrality” or “Bohr-Sommerfeld”-condition imposed on symmetric D-branes becomes more refined (Gawedzki 04).

In summary, the D-brane submanifolds in a Lie group which induce an open string WZW model that a) has one current algebra symmetry and b) is Kapustin-anomaly-free are precisely the conjugacy class-submanifolds GG equipped with a twisted unitary bundle for the restriction of the background B-field to the conjugacy class.

Quantization

on quantization of the WZW model, see at

References

Original references

The Wess-Zumino gauge-coupling term goes back to

and was understood as yielding a 2-dimensional conformal field theory in

and hence (a possible part of) a string theory vacuum/target space in

The WZ term on Σ 2\Sigma_2 was understood in terms of an integral of a 3-form over a cobounding manifold Σ 3\Sigma_3 in

  • Edward Witten, Global aspects of current algebra. Nucl. Phys. B223, 422 (1983)

for the case that Σ 2\Sigma_2 is closed, and generally, in terms of surface holonomy of bundle gerbes/circle 2-bundles with connection in

  • Krzysztof Gawędzki Topological Actions in two-dimensional Quantum Field Theories, in Gerard ’t Hooft et. al (eds.) Nonperturbative quantum field theory Cargese 1987 proceedings, (web)

  • Giovanni Felder , Krzysztof Gawędzki, A. Kupianen, Spectra of Wess-Zumino-Witten models with arbitrary simple groups. Commun. Math. Phys. 117, 127 (1988)

  • Krzysztof Gawędzki, Topological actions in two-dimensional quantum field theories. In: Nonperturbative quantum field theory. ‘tHooft, G. et al. (eds.). London: Plenum Press 1988

as the surface holonomy of a circle 2-bundle with connection. See also the references at B-field and at Freed-Witten anomaly cancellation.

For the fully general understanding as the surface holonomy of a circle 2-bundle with connection see the references below.

Introductions and surveys

An survey of and introduction to the topic is in

  • Patrick Meessen, Strings Moving on Group Manifolds: The WZW Model (pdf)

A classical textbook account in the general context of 2d CFT is

  • P. Di Francesco, P. Mathieu, D. Sénéchal, Conformal field theory, Springer 1997

A basic introduction also to the super-WZW model (and with an eye towards the corresponding 2-spectral triple) is in

A useful account of the WZW model that encompasses both its action functional and path integral quantization as well as the current algebra aspects of the QFT is in

  • Krzysztof Gawedzki, Conformal field theory: a case study in Y. Nutku, C. Saclioglu, T. Turgut (eds.) Frontier in Physics 102, Perseus Publishing (2000) (hep-th/9904145)

This starts in section 2 as a warmup with describing the 1d QFT of a particle propagating on a group manifold. The Hilbert space of states is expressed in terms of the Lie theory of GG and its Lie algebra 𝔤\mathfrak{g}.

In section 4 the quantization of the 2d WZW model is discussed in analogy to that. In lack of a full formalization of the quantization procedure, the author uses the loop algebra 𝓁𝔤\mathcal{l} \mathfrak{g} – the affine Lie algebra – of 𝔤\mathfrak{g} as the evident analog that replaces 𝔤\mathfrak{g} and discusses the Hilbert space of states in terms of that. He also indicates how this may be understood as a space of sections of a (prequantum) line bundle over the loop group.

See also

Relation to gerbes and Chern-Simons theory

Discussion of circle 2-bundles with connection (expressed in terms of bundle gerbes) and discussion of the WZW-background B-field (WZW term) in this language is in

Discussion of how this 2-bundle arises from the Chern-Simons circle 3-bundle is in

and related discussion is in

See also Section 2.3.18 and section 4.7 of

Partition functions

D-branes for the WZW model

A characterization of D-branes in the WZW model as those conjugacy classes that in addition satisfy an integrality (Bohr-Sommerfeld-type) condition missed in other parts of the literature is stated in

The refined interpretation of the integrality condition as a choice of trivialization of the underling principal 2-bundle/bundle gerbe of the B-field over the brane was then noticed in section 7 of

The observation that this is just the special rank-1 case of the more general structure provided by a twisted unitary bundle of some rank nn on the D-brane (gerbe module) which is twisted by the restriction of the B-field to the D-brane – the Chan-Paton gauge field – is due to

The observation that the “multiplicative” structure of the WZW-B-field (induced from it being the transgression of the Chern-Simons circle 3-connection over the moduli stack of GG-principal connections) induces the Verlinde ring fusion product structure on symmetric D-branes equipped with Chan-Paton gauge fields is discussed in

The image in K-theory of these Chan-Paton gauge fields over conjugacy classes is shown to generate the Verlinde ring/the positive energy representations of the loop group in

Relation to dimensional reduction of Chern-Simons

One can also obtain the WZW-model by KK-reduction from Chern-Simons theory.

E.g.

A discussion in higher differential geometry via transgression in ordinary differential cohomology is in

Relation to extended TQFT

Relation to extended TQFT is discussed in

The formulation of the Green-Schwarz action functional for superstrings (and other branes of string theory/M-theory) as WZW-models (and ∞-WZW models) on (super L-∞ algebra L-∞ extensions of) the super translation group is in

Revised on July 14, 2014 00:21:08 by Urs Schreiber (89.204.135.150)