functorial quantum field theory
Reshetikhin?Turaev model? / Chern-Simons theory
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Types of quantum field thories
Where a WZW model is a sigma model quantum field theory whose target space is a group $G$, a parameterization of such a model is a sigma-model (subject usually to some constraints) whose target space now is the total space of a $G$-principal bundle such that the action functional when restricted to fields that take values only in any one fiber, reduces to the given un-parameterized model.
Parameterized WZW models have been argued to provide a more geometric and more complete description of the current algebra-sector of heterotic string backgrounds than the traditional construction in terms of worldsheet fermions (Gates-Siegel 88, Gates-Ketov-Kozenko-Solovev 91, Distler-Sharpe 10, section 7). In particular the Green-Schwarz anomaly of the heterotic string finds a natural interpretation as the obstruction to parameterizing the given WZW term over the given principal bundle:
A (higher) WZW model is an $n$-dimensional sigma-model field theory whose target space is a group $G$ and whose interaction-part of the action functional is the higher holonomy of a circle n-bundle with connection
(whose curvature is given by the global Maurer-Cartan form on $G$).
If one has a $G$-principal bundle
over some base space $B$, then a lift of the structure group to the Heisenberg n-group $Heis_G(\mathbf{L}_{WZW})$ of $\mathbf{L}_{WZW}$ regarded as a prequantum n-bundle, is the structure necessary and sufficient for the fiber-wise copies of $G$ and $\mathbf{L}_{WZW}$ glue fiberwise to a single circle n-bundle with connection
on the total space $X$ of this bundle. This hence yields what one may think of as a coherent collection of WZW models parameterized over base space $B$.
For the case that $G$ is a compact semisimple Lie group and $\mathbf{L}_{WZW}$ its canonical WZW term (the 2-connection on the string 2-group), then
and hence the obstruction to the existence of a parameterization is precisely a string structure, recovering the traditional statement of the Green-Schwarz anomaly (see cwzw for details of this claim).
The heterotic string worldsheet theory is a sigma model on spacetime $X$ combined with a chiral $G$-WZW model for $G = Spin \times E_8 \times E_8$ or similar and with the WZW term being the difference between the differentially twisted looping of the universal Chern-Simons 3-connection of $Spin$ with that of $E_8 \times E_8$.
By (Fiorenza-Rogers-Schreiber 13, section 2.6.1) we find in this case that the Heisenberg 2-group is the string 2-group
It follows thus from the above discussion that a consistent heterotic sigma model requires that the underlying $G$-principal bundle on spacetime admits a string structure. This is the famous Green-Schwarz anomaly cancellation condition, re-expressed as a consistency condition for parameterized WZW models.
On the level of action functionals in codimension 0 this observation is due to (Distler-Sharpe 10, section 7).
By Fiorenza-Sati-Schreiber 13 the super-branes of string theory/M-theory on super Minkowski spacetime $\mathbb{R}^{d-1,1;N}$ classified by the brane scan/the brane bouqet are ∞-Wess-Zumino-Witten theory sigma-model
where $\Gamma$ is the group of periods of the defining super L-∞ algebra L-∞ cocycle
A necessary structure globalizing this to a curved super-spacetime $X$ is a G-structure on $X$ for $G = \mathbf{QuantMorph}(\mathbf{L}_{WZW^{brane}}^{inf})$ the restriction of the WZW term to the infinitesimal disk.
Given this then the Green-Schwarz action functional refines to a local prequantum field theory datum $\mathbf{L}_{WZW^{brane}}$ globally defined on $X$.
(For branes on which other branes may end, such as the D-branes and the M5-brane, here super Minkowski spacetime is replaced by a extended Minkowski super spacetime, as discussed in (Fiorenza-Sati-Schreiber 13)).
More details are in (cwzw).
Parameterized WZW models as sigma models for the heterotic string originate in
Jim Gates, Warren Siegel, Leftons, Rightons, Nonlinear $\sigma$-Models, and Superstrings, Phys.Lett. B206 (1988) 631 (spire)
Jim Gates, Strings, superstrings, and two-dimensional lagrangian field theory, pp. 140-184 in Z. Haba, J. Sobczyk (eds.) Functional integration, geometry, and strings, proceedings of the XXV Winter School of Theoretical Physics, Karpacz, Poland (Feb. 1989), , Birkhäuser, 1989.
Jim Gates, S. Ketov, S. Kozenko, O. Solovev, Lagrangian chiral coset construction of heterotic string theories in $(1,0)$ superspace, Nucl.Phys. B362 (1991) 199-231 (spire)
Discussion relating this to equivariant elliptic genera is in section 7 of
Review of this includes
The corresponding elliptic genera had been considered in
and with more emphasis on equivariant elliptic cohomology in
The discussion of the relevant Heisenberg n-group theory is in
with more details in
Urs Schreiber, Obstruction theory for parameterized higher WZW terms
Urs Schreiber, section “definite forms” in differential cohomology in a cohesive topos
General ∞-Wess-Zumino-Witten theory is set up in section 6 of