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The standard action functional for the higher U(1)-gauge field given by a circle n-bundle with connection $(P, \nabla)$ over a (pseudo) Riemannian manifold $(X,g)$ is
where $F_\nabla$ is the curvature $(n+1)$-form. If the dimension
then the Hodge star operator squares to $+1$ (Lorentian signature) or $-1$ (Euclidean signature) on $\Omega^{k+1}(X)$. Therefore it makes sense in these dimensions to impose the self-duality or chirality constraint
With this duality constraint imposed, one speaks of self-dual higher gauge fields or chiral higher gauge fields or higher gauge fields with self-dual curvature. (These are a higher degree/dimensional generalization of what in Yang-Mills theory are called Yang-Mills instanton field configruations.)
Their quantum field theory is more subtle than usual: first of all the above standard action functional now vanishes constantly.
But sense can be made of the theory of self-dual gauge fields by other means. Notably – by a version of the holographic principle the – partition function of the self-dual theory on an $X$ of dimension $4 k +2$ is given by the state (wave function) of an abelian higher Chern-Simons theory in dimension $4 k + 3$.
Generally, higher dimensional Chern-Simons theory in dimension $4k+3$ (for $k \in \mathbb{N}$) is holographically related to self-dual higher gauge theory in dimension $4k+2$ (at least in the abelian case).
$(k=0)$: ordinary 3-dimensional Chern-Simons theory is related to a string sigma-model on its boundary;
$(k=1)$: 7-dimensional Chern-Simons theory is related to a fivebrane model on its boundary;
$(k=2)$: 11-dimensional Chern-Simons theory is related to a parts of a type II string theory on its bounday (or that of the space-filling 9-brane, if one wishes) (BelovMoore).
We indicate why higher dimensional Chern-Simons theory is – if holographically related to anything – holographically related to self-dual higher gauge theory.
The phase space of higher dimensional Chern-Simons theory in dimension $4k+3$ on $\Sigma \times \mathbb{R}$ can be identified with the space of flat $2k+1$-forms on $\Sigma$. The presymplectic form on this space is given by the pairing
obtained as the integration of differential forms over $\Sigma$ of the wedge product of the two forms.
The geometric quantization of the theory requires that we choose a polarization of the complexification of this space (split the space of forms into “coordinates” and their “canonical momenta”).
One way to achieve this is to choose a conformal structure on $\Sigma$. The corresponding Hodge star operator
provides the polarization by splitting into self-dual and anti-self-dual forms:
notice that (by the formulas at Hodge star operator) we have on mid-dimensional forms
Therefore it provides a complex structure on $\Omega^{2k+1}(\Sigma) \otimes \mathbb{C}$.
We see that the symplectic structure on the space of forms can equivalently be rewritten as
Here on the right now the Hodge inner product of $B_1$ with $\star B_2$ appears, which is invariant under applying the Hodge star to both arguments.
We then decompose $\Omega^{2k+1}(\Sigma)$ into the $\pm i$-eigenspaces of $\star$: say $B \in \Omega^{2k+1}(\Sigma)$ is imaginary self-dual if
and imaginary anti-self-dual if
Then for imaginary self-dual $B_1$ and $B_2$ we find that the symplectic pairing is
Therefore indeed the symplectic pairing vanishes on the self-dual and on the anti-selfdual forms. Evidently these provide a decomposition into Lagrangian subspaces.
Therefore a state of higher Chern-Simons theory on $\Sigma$ may locally be thought of as a function of the self-dual forms on $\Sigma$. Under holography this is (therefore) identified with the correlator of a self-dual higher gauge theory on $\Sigma$.
By the above discussion (…) the partition function of self-dual higher gauge theory is given by (a multiple of) the unique holomorphic section of a square root of the line bundle classified by the secondary intersection pairing. (Witten I, Hopkins-Singer).
(…)
The simplest case. A review is for instance in (Witten I, section 2). A detailed discussion is in (GBMNV).
(…)
The worldvolume theory of the M5-brane, the 6d (2,0)-superconformal QFT, contains a self-dual 2-form field. Its AdS7-CFT6 holographic description by 7-dimensional Chern-Simons theory is due to (Witten I).
The RR-field in type II string theory are self-dual (as a formal sum of fields). A holographic description is discussed in (Belov-Moore II).
The following table lists classes of examples of square roots of line bundles
An survey and introduction is in
Original reference on self-dual/chiral fields include
X. Bekaert, Marc Henneaux, Comments on Chiral $p$-Forms (arXiv:hep-th/9806062)
Mans Henningson, Bengt E.W. Nilsson, Per Salomonson, Holomorphic factorization of correlation functions in (4k+2)-dimensional (2k)-form gauge theory (arXiv:hep-th/9908107)
M. Henningson, The quantum Hilbert space of a chiral two-form in $d = 5 + 1$ dimensions (arxiv:hep-th/0111150)
The chiral boson in 2d is discussed in detail in
A quick exposition of the basic idea is in
A precise formulation of the phenomenon in terms of ordinary differential cohomology is given in
Dan Freed, Greg Moore, Graeme Segal,
The Uncertainty of Fluxes Commun.Math.Phys.271:247-274 (2007) (arXiv:hep-th/0605198)
Heisenberg Groups and Noncommutative Fluxes , AnnalsPhys.322:236-285 (2007) (arXiv:hep-th/0605200)
The idea of describing self-dual higher gauge theory by holography with abelian higher dimensional Chern-Simons theory in one dimension higher originates in
Conceptual aspects of this are also discussed in section 6.2 of
Motivated by this the ordinary differential cohomology of self-dual fields had been discussed in
The generalization of this to generalized differential cohomology is discussed from p. 26 on in
More discussion of the general principle is in
The application of this to the description of type II string theory in 10-dimensions to 11-dimensional Chern-Simons theory is in the followup
Discussion of the quantum anomaly of self-dual theories is in
Samuel Monnier, The anomaly line bundle of the self-dual field theory (arXiv:1109.2904)
Samuel Monnier, The global gravitational anomaly of the self-dual field theory (arXiv:1110.4639, pdf slides)
Discussion of the conformal blocks and geometric quantization of self-dual higher gauge theories is in
Kiyonori Gomi, An analogue of the space of conformal blocks in $(4k+2)$-dimensions (pdf)
Samuel Monnier, Geometric quantization and the metric dependence of the self-dual field theory (arXiv:1011.5890)
For the case of nonabelian self-dual 1-form gauge fields see the references at Yang-Mills instanton.