enhanced gauge symmetry



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Generally, an enhanced gauge symmetry referes to the appearance of gauge theory with a larger gauge group than may be suprficially apparent.

On coincident D-branes

A famous case is the enhance gauge symmetry in the Chan-Paton gauge fields of coincident D-branes. A priori each D-brane carries a complex line bundle/circle principal bundle, but as nn of them coincide the gauge group is expected to enhance from (U(1)) n(U(1))^n to the unitary group U(n)U(n) (or the special unitary group SU(n)SU(n)).

Mathematically this gauge enhancement on D-branes is modeled by the Baum-Douglas geometric cycle model for K-homology (Reis-Szabo 05, def. 1.5 and p. 16, Szabo 08, p. 4): One such cycle is given by a submanifold WϕXW \overset{\phi}{\hookrightarrow} X of spacetime XX equipped with spin^c structure and with a complex vector bundle EXE \to X, and given two such cycles (W,ϕ,E 1)(W,\phi,E_1) and (W,ϕ,E 2)(W,\phi,E_2) with the same underlying manifold (“coincident D-brane worldvolume”) then their formal direct sum is identified with the single cycle carrying the direct sum of vector bundles

(W,ϕ,E 1)+(W,ϕ,E 2)(W,ϕ,E 1E 2). (W,\phi, E_1) + (W, \phi, E_2) \;\sim\; (W, \phi, E_1 \oplus E_2) \,.

If here E iE_i has rank n in_i and hence structure group U(n i)U(n_i), then the direct sum of vector bundles E 1E 2E_1 \oplus E_2 has structure group the product group U(n 1)×U(n 2)U(n_1) \times U(n_2).

In particular the sum of nn coincidnt cycles each carrying a line bundle yields a rank-nn vector bundle with structure group (U(1)) n(U(1))^n. In this sense then a cycle carrying a general rank nn-vector bundle, hence with structure group U(n)U(n), may be thought of as a “gauge enhancement” of making nn D-branes coincide.

From massless BPS states

In the context of the realization of super Yang-Mills theories as effective field theories of superstring theory, BPS branes in the string background becomes BPS states of the SYM theory. These become massless as the cycles on the which BPS charge is supported shrink away (at boundary points of the given moduli space). In this case these additional massless states may appear as additional gauge bosons in the compactified gauge theory which thereby develops a bigger gauge group. Typically a previously abelian gauge group becomes non-abelian this way.

Examples include KK-compactification of M-theory on K3 fibers and on G2-manifolds with ADE singularities, corresponding to F-theory on elliptic fibrations making K3-fibrations with singular fibers, corresponding to heterotic string theory on singular elliptic fibrations. See at

On M9-Branes

In Horava-Witten theory there is supposed to be gauge enhancement over the M9-branes such as to identify their worldvolume theory with E8-heterotic string theory.

Under the duality between M-theory and type IIA string theory the M9-brane may be identified with the O8-plane:

from GKSTY 02, section 3

This may be used to understand the gauge enhancement to E8-gauge groups at the heterotic boundary of Horava-Witten theory:

from GKSTY 02, section 3


On coincident D-branes

The idea that on NN coincident D-branes there is gauge enhancement to U(N)U(N)-gauge field theory is due to

There, this is called an “obvious guess” (first line on p. 8). Subsequently, most authors cite this obvious guess as a fact; for instance the review

By actual computation in open string field theory “convincing evidence” (see bottom of p. 22) was found, numerically, in

Similar numerical derivation, as well as exact derivation at zero momentum, is in

The first full derivation seems to be due to

  • Taejin Lee, Covariant open bosonic string field theory on multiple D-branes in the proper-time gauge, Journal of the Korean Physical Society December 2017, Volume 71, Issue 12, pp 886–903 (arXiv:1609.01473)

which is surveyed in

That on D0-branes this reproduces the BFSS matrix model and on D(-1)-branes the IKKT matrix model is shown in

Discussion of gauge enhancement on coincident D-branes in terms of Baum-Douglas geometric cycles for K-homology is in

  • Rui Reis, Richard Szabo, Geometric K-Homology of Flat D-Branes ,Commun.Math.Phys. 266 (2006) 71-122, Journal of the Korean Physical Society December 2017, Volume 71, Issue 12, pp 886–903 (arXiv:hep-th/0507043)

reviewed in

On M-branes at ADE-singularities

The picture of M-theory lift of gauge enhancement on D6-branes is due to

Review of the standard lore of gauge enhancement in M-theory includes



Original articles include

Discussion via duality of M9-branes to O-planes is in

  • E. Gorbatov, V.S. Kaplunovsky, J. Sonnenschein, Stefan Theisen, S. Yankielowicz, section 3 of On Heterotic Orbifolds, M Theory and Type I’ Brane Engineering, JHEP 0205:015, 2002 (arXiv:hep-th/0108135)

Last revised on May 23, 2018 at 17:16:39. See the history of this page for a list of all contributions to it.