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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeMay 13th 2013
    • (edited May 13th 2013)

    am in the process of adding some notes on how the D=5 super Yang-Mills theory on the worldvolume of the D4-brane is the double dimensional reduction of the 6d (2,0)-superconformal QFT in the M5-brane.

    started a stubby double dimensional reduction in this context and added some first further pointers and references to M5-brane, to D=5 super Yang-Mills theory and maybe elsewhere.

    But this still needs more details to be satisfactory, clearly.

    • CommentRowNumber2.
    • CommentAuthorDavid_Corfield
    • CommentTimeMay 13th 2013

    What happens to the S-duality connected to 6d (2,0)-superconformal QFT when undergoing this reduction? What happens to holographic duals when one is reduced?

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeMay 13th 2013
    • (edited May 13th 2013)

    This is I think part of the original Witten and Kapustin-Witten story on geometric Langlands. Roughly like this:

    • the 6d (2,0)(2,0)-superconformal QFT on the worldvolume of the M5-brane in 11-d SuGra has a conformal invariance, specifically Moebius transformations when taken to be a product of a 4d space with a torus

    • double dimensional reduction makes this the 5d super-Yang-Mills theory on the worldvolume of the D4-brane in 10-d SuGra

    • further ordinary dimensional reduction of the 5d worldvolume theory to a 4d theory yields 4d Yang-Mills and its topological twists. Now the Montonen-Olive S-duality of that theory is supposed to be the shadow of the original conformal invariance of the (2,0)-theory on the torus which was “dimensionally reduced”.

    • further compactifying down to d=2 turns this into geometric Langlands duality.

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeDec 16th 2015

    added to double dimensional reduction a formal definition for double dimensional reduction of cocycles in differential cohomology.

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeJan 19th 2017

    Meanwhile we have a much more sophisticated formulation of double dimensional reduction. It’s not reflected in the entry yet. But I am writing an exposition as talk notes here.

    • CommentRowNumber6.
    • CommentAuthorDavid_Corfield
    • CommentTimeJan 21st 2017

    Expressed in HoTT, I imagine that could look beautifully simple.

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeFeb 14th 2017
    • (edited Feb 14th 2017)

    I have moved over (here) at least statement and proof of the abstract \infty-topos theoretic formulation of double dimensional reduction, in the following form:

    Let H\mathbf{H} be any (∞,1)-topos and let GG be an ∞-group in H\mathbf{H}. There is a pair of adjoint ∞-functors of the form

    H[G,]/GhofibH /BG, \mathbf{H} \underoverset {\underset{[G,-]/G}{\longrightarrow}} {\overset{hofib}{\longleftarrow}} {\bot} \mathbf{H}_{/\mathbf{B}G} \,,


    Hence for

    then there is a natural equivalence

    H(X^,A)originalfluxesoxidationreductionH(X,[G,A]/G)doublydimensionally reducedfluxes \underset{ \text{original} \atop \text{fluxes} }{ \underbrace{ \mathbf{H}(\hat X\;,\; A) } } \;\; \underoverset {\underset{oxidation}{\longleftarrow}} {\overset{reduction}{\longrightarrow}} {\simeq} \;\; \underset{ \text{doubly} \atop { \text{dimensionally reduced} \atop \text{fluxes} } }{ \underbrace{ \mathbf{H}(X \;,\; [G,A]/G) } }

    given by

    (X^A)(X [G,A]/G BG) \left( \hat X \longrightarrow A \right) \;\;\; \leftrightarrow \;\;\; \left( \array{ X && \longrightarrow && [G,A]/G \\ & \searrow && \swarrow \\ && \mathbf{B}G } \right)
    • CommentRowNumber8.
    • CommentAuthorUrs
    • CommentTimeJan 11th 2019
    • (edited Jan 11th 2019)

    I have taken the liberty of adding pointers to our formalization of double dimensional reduction:

    Formalization of double dimensional reduction is discussed in rational homotolpy theory in

    and in full homotopy theory in

    Exposition is in

    diff, v17, current

    • CommentRowNumber9.
    • CommentAuthorDavid_Corfield
    • CommentTimeJan 11th 2019

    Typo fixed in link.

    diff, v18, current

    • CommentRowNumber10.
    • CommentAuthorUrs
    • CommentTimeDec 9th 2022

    Added the remark that the differential geometry of the double dimensional reduction of the M2-brane- and M5-brane-charges was maybe first clearly written out in:

    diff, v29, current

    • CommentRowNumber11.
    • CommentAuthorperezl.alonso
    • CommentTimeOct 6th 2023
    • (edited Oct 6th 2023)

    One can regard Proposition 2.3 to be a statement about the double-dimensional reduction of FieldsFields. But there should be a more general statement concerning the reduction of theories. For instance, if FieldsFields carries a prequantum circle n-bundle (e.g. take Fields=BG connFields =BG_{conn} equipped with a characteristic class, say the Chern-Simons 3-bundle to B 3U(1)B^3U(1)), then one should be able to do double-dimensional reduction not only of the fields but also of the corresponding theory. This I guess will involve some statement similar to Proposition 2.3 involving a more general comma category. Is this sorted out anywhere? I’m particularly asking because there have been some recent papers arguing one can get Symmetry TFT’s from doing dimensional reduction “of” M-theory, so it would be nice to make this precise.

    • CommentRowNumber12.
    • CommentAuthorUrs
    • CommentTimeOct 7th 2023
    • (edited Oct 7th 2023)

    Just to note that the double dimensional reduction by cyclification does not involve truncation:

    The space/stack of dd-reduced fields is still equivalent to that of the original fields (the “reduction/oxidation” equivalence).

    In that sense there is no necessary cause to change the prequantum \infty-bundle.

    And if one does want to truncate some of the KK-modes, that would typically amount to restricting to a sub-stack of the dd-reduced fields, for which the appropriate prequantum bundle would just be the restriction (pullack along the inclusion) of the original one.

    • CommentRowNumber13.
    • CommentAuthorperezl.alonso
    • CommentTimeApr 4th 2024

    Is it described anywhere how this process leads to getting WZW from CS on a Σ×S 1\Sigma\times S^1 worldvolume? That is, how one uses this to go from Σ×S 1BGB 3U(1) conn\Sigma\times S^1\to BG \to B^3 U(1)_{conn} to ΣGB 2U(1) conn\Sigma\to G\to B^2 U(1)_{conn} where in the first case one has the CS 2-gerbe and in the second case it is the WZW 1-gerbe?

    • CommentRowNumber14.
    • CommentAuthorUrs
    • CommentTimeApr 5th 2024

    Yes, this is pp. 40 in arXiv:1304.0236, following arXiv:1207.5449 (pp. 3) and arXiv:0410013.

    • CommentRowNumber15.
    • CommentAuthorperezl.alonso
    • CommentTimeApr 5th 2024

    Thanks. A couple of questions. If you start with an action GAut( CS)G\to \text{Aut}(\nabla_{CS}) of a higher group GG on Chern-Simons, what happens to this action when passing to WZW? And also, in p.41 it is mentioned that the sections of the WZW gerbe are twisted unitary bundles, referencing Example 2.5.4 for this, which does not talk about gerbes with connection. But isn’t the fact that the sections of a line bundle gerbe with connection 𝒢\mathcal{G} can be expressed as bundle gerbe morphisms from a trivial gerbe ω\mathcal{I}_{\omega} to 𝒢\mathcal{G}, that is as twisted vector bundles with connection, only valid for torsion Dixmier-Douady classes?

    • CommentRowNumber16.
    • CommentAuthorUrs
    • CommentTimeApr 5th 2024

    Right, the twisted bundles discussed there are finite-rank, hence the corresponding gerbe must have torsion DD-class.

    But this is the case after pullback to D-brane worldvolume submanifolds QGQ \hookrightarrow G as considered there, if by Freed-Witten anomaly cancellation and assuming Spin c\mathrm{Spin}^c-structure, the restriction of the class of the gerbe to the brane worldvolume QQ is torsion.

    On the other hand, in a more sophisticated perspective the quantization is anyways described directly by pushforward in K-theory twisted by that gerbe, as in section 5.2.4 of Joost Nuiten’s thesis (pp. 115).