In theoretical physics and mathematics, a Wess–Zumino–Witten (WZW) model, also called a Wess–Zumino–Novikov–Witten model, is a type of two-dimensional conformal field theory named after Julius Wess, Bruno Zumino, Sergei Novikov and Edward Witten. A WZW model is associated to a Lie group (or supergroup), and its symmetry algebra is the affine Lie algebra built from the corresponding Lie algebra (or Lie superalgebra). By extension, the name WZW model is sometimes used for any conformal field theory whose symmetry algebra is an affine Lie algebra.
Geometrical interpretation of the Wess–Zumino term
If e<sub>a</sub> are the basis vectors for the Lie algebra, then <math>\mathcal{K} (e_a, [e_b, e_c])</math> are the structure constants of the Lie algebra. The structure constants are completely anti-symmetric, and thus they define a 3-form on the group manifold of G. Thus, the integrand above is just the pullback of the harmonic 3-form to the ball <math>\mathbf{B}^3.</math> Denoting the harmonic 3-form by c and the pullback by <math>\gamma^*,</math> one then has
:<math>S^{\mathrm WZ}(\gamma) = \int_{\mathbf{B}^3} \gamma^{*} c.</math>
This form leads directly to a topological analysis of the WZ term.
Geometrically, this term describes the torsion of the respective manifold. The presence of this torsion compels teleparallelism of the manifold, and thus trivialization of the torsionful curvature tensor; and hence arrest of the renormalization flow, an infrared fixed point of the renormalization group, a phenomenon termed geometrostasis.
Symmetry algebra
Generalised group symmetry
The Wess–Zumino–Witten model is not only symmetric under global transformations by a group element in <math> G </math>, but also has a much richer symmetry. This symmetry is often called the <math> G(z) \times G(\bar{z}) </math> symmetry. Namely, given any holomorphic <math>G</math>-valued function <math>\Omega(z)</math>, and any other (completely independent of <math>\Omega(z)</math>) antiholomorphic <math>G</math>-valued function <math>\bar{\Omega}(\bar{z})</math>, where we have identified <math> z=x+iy </math> and <math> \bar{z} = x-iy </math> in terms of the Euclidean space coordinates <math>x,y</math>, the following symmetry holds:
:<math> S_k (\gamma) = S_k (\Omega \gamma \bar{\Omega}^{-1} ) </math>
One way to prove the existence of this symmetry is through repeated application of the Polyakov–Wiegmann identity regarding products of <math>G</math>-valued fields:
:<math> S_k (\alpha \beta^{-1}) = S_k(\alpha) + S_k(\beta^{-1}) + \frac{k}{16\pi^2}\int d^2 x \textrm{Tr}(\alpha^{-1} \partial_{\bar{z \alpha \beta^{-1} \partial_{z} \beta) </math>
The holomorphic and anti-holomorphic currents <math> J(z) = - \frac{1}{2}k (\partial_z \gamma) \gamma^{-1} </math> and <math> \bar{J}(\bar{z}) = - \frac{1}{2} k \gamma^{-1} \partial_{\bar{z \gamma </math> are the conserved currents associated with this symmetry. The singular behaviour of the products of these currents with other quantum fields determine how those fields transform under infinitesimal actions of the <math> G(z) \times G(\bar{z})</math> group.
Affine Lie algebra
Let <math>z</math> be a local complex coordinate on <math>\Sigma</math>,
<math> \{t^a\} </math> an orthonormal basis (with respect to the Killing form) of the Lie algebra of <math>G </math>, and <math> J^a(z) </math> the quantization of the field <math> \mathcal{K}(t^a,\partial_z g g^{-1}) </math>. We have the following operator product expansion:
: <math> J^a(z) J^b(w) = \frac{k\delta^{ab{(z - w)^2} + \frac{i f^{ab}_c J^c(w)}{z - w} + \mathcal{O}(1), </math>
where <math>f^{ab}_c</math> are the coefficients such that <math>[t^a,t^b] = f^{ab}_c t^c</math>.
Equivalently, if <math> J^a(z) </math> is expanded in modes
: <math> J^a(z) = \sum_{n \in \mathbb{Z J_n^a z^{-n-1}, </math>
then the current algebra generated by <math> \{J_n^a\} </math> is the affine Lie algebra associated to the Lie algebra of <math>G</math>, with a level that coincides with the level <math>k</math> of the WZW model.
If <math>\mathfrak{g}=\mathrm{Lie}(G)</math>, the notation for the affine Lie algebra is <math> \hat{\mathfrak{g_k</math>.
The commutation relations of the affine Lie algebra are
: <math> [J^a_n,J^b_m] = f^{ab}_c J^c_{m+n} + kn\delta^{ab}\delta_{n+m,0}. </math>
This affine Lie algebra is the chiral symmetry algebra associated to the left-moving currents <math> \mathcal{K}(t^a,\partial_z g g^{-1}) </math>. A second copy of the same affine Lie algebra is associated to the right-moving currents <math>\mathcal{K}(t^a, g^{-1}\partial_{\bar z} g) </math>. The generators <math>\bar J^a(z)</math> of that second copy are antiholomorphic. The full symmetry algebra of the WZW model is the product of the two copies of the affine Lie algebra.
Sugawara construction
The Sugawara construction is an embedding of the Virasoro algebra into the universal enveloping algebra of the affine Lie algebra. The existence of the embedding shows that WZW models are conformal field theories. Moreover, it leads to Knizhnik–Zamolodchikov equations for correlation functions.
The Sugawara construction is most concisely written at the level of the currents: <math> J^a(z) </math> for the affine Lie algebra, and the energy-momentum tensor <math>T(z)</math> for the Virasoro algebra:
: <math> T(z) = \frac{1}{2(k + h^{\vee})} \sum_a : J^a J^a : (z), </math>
where the <math> : </math> denotes normal ordering, and <math>h^{\vee}</math> is the dual Coxeter number. By using the OPE of the currents and a version of Wick's theorem one may deduce that the OPE of <math> T(z) </math> with itself is given by
and also in more complicated supergroups such as <math>G=PSU(1,1|2)</math>.
Non-factorizable representations are responsible for the fact that the corresponding WZW models are logarithmic conformal field theories.
Other theories based on affine Lie algebras
The known conformal field theories based on affine Lie algebras are not limited to WZW models.
For example, in the case of the affine Lie algebra of the <math>SU(2)</math> WZW model, modular invariant torus partition functions obey an ADE classification, where the <math>SU(2)</math> WZW model accounts for the A series only. The D series corresponds to the <math>SO(3)</math> WZW model, and the E series does not correspond to any WZW model.
Another example is the <math>H_3^+</math> model. This model is based on the same symmetry algebra as the <math>SL(2,\mathbb{R})</math> WZW model, to which it is related by Wick rotation. However, the <math>H_3^+</math> is not strictly speaking a WZW model, as <math>H_3^+ =SL(2,\mathbb{C})/SU(2)</math> is not a group, but a coset.
Fields and correlation functions
Fields
Given a simple representation <math>\rho</math> of the Lie algebra of <math>G</math>, an affine primary field <math>\Phi^\rho(z)</math> is a field that takes values in the representation space of <math>\rho</math>, such that
:<math>
J^a(y) \Phi^\rho(z) = -\frac{\rho(t^a)\Phi^\rho(z)}{y-z} + O(1)\ .
</math>
An affine primary field is also a primary field for the Virasoro algebra that results from the Sugawara construction. The conformal dimension of the affine primary field is given in terms of the quadratic Casimir <math>C_2(\rho)</math> of the representation <math>\rho</math> (i.e. the eigenvalue of the quadratic Casimir element <math>K_{ab}t^at^b</math> where <math>K_{ab}</math> is the inverse of the matrix <math>\mathcal{K}(t^a,t^b)</math> of the Killing form) by
:<math>
\Delta_\rho = \frac{C_2(\rho)}{2(k+h^\vee)}\ .
</math>
For example, in the <math>SU(2)</math> WZW model, the conformal dimension of a primary field of spin <math>j</math> is
:<math>\Delta_j = \frac{j(j+1)}{k+2} \ .</math>
By the state-field correspondence, affine primary fields correspond to affine primary states, which are the highest weight states of highest weight representations of the affine Lie algebra.
Correlation functions
If the group <math>G</math> is compact, the spectrum of the WZW model is made of highest weight representations, and all correlation functions can be deduced from correlation functions of affine primary fields via Ward identities.
If the Riemann surface <math>\Sigma</math> is the Riemann sphere, correlation functions of affine primary fields obey Knizhnik–Zamolodchikov equations. On Riemann surfaces of higher genus, correlation functions obey Knizhnik–Zamolodchikov–Bernard equations, which involve derivatives not only of the fields' positions, but also of the surface's moduli.
Gauged WZW models
Given a Lie subgroup <math>H\subset G</math>, the <math>G/H</math> gauged WZW model (or coset model) is a nonlinear sigma model whose target space is the quotient <math>G/H</math> for the adjoint action of <math>H</math> on <math>G</math>. This gauged WZW model is a conformal field theory, whose symmetry algebra is a quotient of the two affine Lie algebras of the <math>G</math> and <math>H</math> WZW models, and whose central charge is the difference of their central charges.
Applications
The WZW model whose Lie group is the universal cover of the group <math>\mathrm{SL}(2,\R)</math> has been used by Juan Maldacena and Hirosi Ooguri to describe bosonic string theory on the three-dimensional anti-de Sitter space <math>AdS_3</math>.
The <math>SL(2,\mathbb{R})/U(1)</math> gauged WZW model has an interpretation in string theory as Witten's two-dimensional Euclidean black hole.
The same model also describes certain two-dimensional statistical systems at criticality, such as the critical antiferromagnetic Potts model.