What is the global symmetry group associated to the C-field?
Answers
The C-field in 11-dimensional supergravity is an elusive object that is not the simple higher U(1)-gauge field one would naively make this out to be. For an overview of possible models for this object, see, for instance, section 3 of "The M-theory 3-form and E8 gauge theory" by Diaconescu, Freed and Moore (henceforth DFM).
However, it is always an object that carries with it a notion of "gauge transformation", and for naive higher U(1)-gauge fields with a transformation law
C↦C+dΛ
for C a p-form and Λ a (p−1)-form, one can easily see that there are "gauge transformations" that actually do not change the gauge field at all - those with dΛ=0. However, objects charged under this U(1) would transform by eiΛ, meaning this transformation is non-trivial on the other fields. This means that there is a global symmetry group associated to this gauge transformation law, namely given by all closed (p−1)-forms Cp−1dR(M) on our spacetime manifold M that does not change the gauge field (hence does not need to be quotiented out in order to "fix the gauge") and therefore remains even after quantization. If we consider that the objects charged under such a symmetry are (p−1)-dimensional objects, it is clear that the proper symmetry operator is ei∫ΣΛ where Σ is the charge object, and so the final global symmetry group is in fact Hp−1(C,U(1)) since exact forms just act as the identity.1
A similar reasoning seems to be carried out in "M-Theory Dynamics On A Manifold Of G2 Holonomy" by Atiyah and Witten to obtain the total and unbroken global symmetry groups associated to the C-field. However, as I mentioned in the first paragraph of this question, the C-field is not a simple higher gauge field, and its exact "gauge group" is a subtle question.
For instance, in one of DFM's models, the proper notion of a gauge transformation is that "the C-field" is a pair of objects (A,c) where A is an ordinary E8-gauge field and c a 3-form with integral periods, and the gauge transformations are given by
A↦A+αc↦c−CS3(A,A+α)+ω,
where CS3(A,A+α) is the relative Chern-Simons invariant 3-form between two connections given by integrating tr(F2) along the straight line between A and A+α in connection space (which is affine, so this is possible). The α is simply a 1-form on ad(P) and ω is a 3-form with integral periods.
There seems to be no evident notion of how such a transformation acts on objects charged under the C-field, nor do there seem to be global transformations in this case or indeed a straightforward relation of this transformation to the Λ considered earlier. Section 7 of DFM defines the proper notion of charge for the E8-model of the C-field, but does not consider how objects charged thusly transform as far as I can see.
What is the proper action of such a gauge transformation in the sense of DFM on charged objects? How can this be reconciled with the analysis of global symmetries of the C-field as done by Atiyah and Witten? Is there a notion of the global symmetries associated to the gauge symmetry of the C-field in the stricter formulation where it is no longer a naive higher U(1)-gauge field? Think about these........