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A gauge theory may denote either a classical field theory or a quantum field theory whose field configurations are cocycles in differential cohomology (abelian or nonabelian).
An ordinary gauge theory is a quantum field theory whose field configurations are vector bundles with connection.
This includes notable the fields that carry the three fundamental forces of the standard model of particle physics:
Ordinary electromagnetism in the absence of magnetic charges is a gauge theory of $U(1)$-principal bundles with connection.
Fields in Yang-Mills theory (such as appearing in the standard model of particle physics and in GUTs) are vector bundles with connection.
Other examples include formal physical models.
The group $G$ in these examples is called the gauge group of the theory.
The above examples of gauge fields consisted of cocycles in degree-$1$ differential cohomology.
More generally, a higher gauge theory is a quantum field theory whose field configurations are cocycles in more general differential cohomology, for instance higher degree Deligne cocycles or more generally cocycles in other differential refinements, such as in differential K-theory.
This generalization does contain experimentally visible physics such as
But a whole tower of higher and generalized gauge theories became visible with the study of higher supergravity theories,
The Kalb-Ramond field is a bundle gerbe with connection, a Deligne cocycle with curvature 3-form.
The supergravity C-field is a Deligne cocycle with curvature 4-form.
The RR-field is a cocycle in differential K-theory.
In the first order formulation of gravity also the theory of gravity looks a little like a gauge theory. However, there is a crucial difference. What really happens here is Cartan geometry: the field of gravity may be encoded in a vielbein field, namelely an orthogonal structure on the tangent bundle, hence as an example of a G-structure, and the torsion freedom of this G-structure may be encoded by an auxiliary connection, namely a Cartan connection, often called the “spin connection” in this context. Hence while in the formulation of Cartan geometry gravity is described by many of the ingredients from differential geometry that also govern pure gauge theory, it’s not quite the same. In particular there is a constraint on a Cartan connection, which in terms of vielbein fields is the contraint that the vielbein (which is part of the Cartan connection) is non-degenerate, and hence really a “soldering form”. Such a constraint is absent in a “genuine” gauge theory such as Yang-Mills theory or Chern-Simons theory.
Sometimes one see the view expressed that gauge symmetry is “just a redundancy” in the description of a theory of physics, for instance in that among observables it is only the gauge invariant ones which are physically meaningful.
This statement however
In the presence of magnetic charge (and then even in the absence of chiral fermion anomalies?) the standard would-be action functional for higher gauge theories may be ill-defined. The Green-Schwarz mechanism is a famous phenomenon in differential cohomology by which such a quantum anomaly cancels against that given by chiral fermions.
The following tries to give an overview of some collection of gauge fields in physics, their models by differential cohomology and further details.
cocycle in lowest degree nonabelian differential cohomology
originally realized in terms of differential ?ech cocycles
with coefficients in the groupoid of Lie-algebra valued forms,
then traditionally in terms of vector bundles with connection
field strength depending on the group $G$ we have
$G = U(1)$ - electromagnetism (see below)
$G = SU(2)\times U(1)$ - electroweak force field strength
$G = SU(3)$ - strong nuclear force field
cocycle in degree-$2$ ordinary differential cohomology
field strength: the electric field $E$ and magnetic field $B$, locally at a point $x \in X$
on $X = \mathbb{R}^3\backslash \{0\}$: underlying class in integral cohomology $cl(\hat F) \in H(X,\mathbf{B} U(1)) \simeq H^2(X,\mathbb{Z})$ is the magnetic charge
parallel transport: gauge interaction piece of action functional of the electrically charged quantum 1-particle
cocycle in degree-$3$ ordinary differential cohomology
naturally/historically realized in terms of
a cocycle in ?ech–Deligne cocycle
a bundle gerbe with connection
field strength: $H \in \Omega^3(X)$ the “$H$-field” – on a D-brane this is the magnetic current for the Yang-Mills field on the brane
parallel transport: gauge interaction piece of action functional of the electrically charged quantum 2-particle (the string).
cocycle in degree-$4$ ordinary differential cohomology
naturally/historically realized in terms of as a cocycle in ?ech–Deligne cocycle
using the D'Auria-Fre formulation of supergravity it may also be thought of as a nonabelian differential cocycle given by a Cartan-Ehresmann ∞-connection
field strength: $H \in \Omega^4(X)$ the “$G$-field” – in heterotic supergravity this is the 5-brane magnetic current for the twisted Kalb-Ramond field
parallel transport: gauge interaction piece of action functional of the electrically charged quantum 3-particle (the membrane).
cocycle in differential K-theory
field strength: RR-forms
gauge field: models and components
An introduction to concepts in the quantization of gauge theories is in
A standard textbook on the BV-BRST formalism for the quantization of gauge systems is in
Discussion of abelian higher gauge theory in terms of differential cohomology is in
Dan Freed, Dirac charge quantization and generalized differential cohomology
Alessandro Valentino, Differential cohomology and quantum gauge fields (pdf)
José Figueroa-O'Farrill, Gauge theory (web page)
Tohru Eguchi, Peter Gilkey, Andrew Hanson, Gravitation, gauge theories and differential geometry, Physics Reports 66:6 (1980) 213—393 pdf
For discussion in the context of gravity see also
Standard discussion of gauge theory in the context of algebraic quantum field theory (AQFT) includes
For AQFT on curved spacetimes the axioms of AQFT need to be promoted to a context of higher geometry unless locality is broken, see the expositions at
Alexander Schenkel, On the problem of gauge theories in locally covariant QFT, talk at Operator and Geometric Analysis on Quantum Theory Trento, 2014 (web)
This was established in
and the program of improving the axioms of AQFT on curved spacetimes to the stacky context in order to accomodate gauge theory includes the following articles:
Marco Benini, Alexander Schenkel, Richard Szabo, Homotopy colimits and global observables in Abelian gauge theory (arXiv:1503.08839)
Marco Benini, Alexander Schenkel, Quantum field theories on categories fibered in groupoids (arXiv:1610.06071)
Marco Benini, Alexander Schenkel, Urs Schreiber, The stack of Yang-Mills fields on Lorentzian manifolds (arXiv:1704.01378)
An exposition of the relation to geometric Langlands duality is in
A discussion of “gauge” and gauge transformation in metaphysics is in
Hermann Weyl’s historical argument motivating gauge theory in physics from rescaling of units of length was given in 1918 in
Hermann Weyl, Raum, Zeit, Materie: Vorlesungen über die Allgemeine Relativitätstheorie, Springer Berlin Heidelberg 1923
The manuscript of Weyl’s first book on mathematical physics, Space – Time – Matter (STM) (Raum – Zeit – Materie), delivered to the publishing house (Springer) Easter 1918, did not contain Weyl’s new geometry and proposal for a UFT. It was prepared from the lecture notes of a course given in the Summer semester of 1917 at the Polytechnical Institute (ETH) Zürich. Weyl included his recent findings only in the 3rd edition (1919) of the book. The English and French versions (Weyl 1922b, Weyl 1922a), translated from the fourth revised edition (1921), contained a short exposition of Weyl’s generalized metric and the idea for a scale gauge theory of electromagnetism. (Scholz)
See
Quick reviews include
More comprehensive historical accounts include
Lochlainn O'Raifeartaigh, The Dawning of Gauge Theory, Princeton University Press (1997)
Lochlainn O'Raifeartaigh, Norbert Straumann, Gauge Theory: Historical Origins and Some Modern Developments Rev. Mod. Phys. 72, 1-23 (2000).
Norbert Straumann, Early History of Gauge Theories and Weak Interactions (arXiv:hep-ph/9609230)
Norbert Straumann, Gauge principle and QED, talk at PHOTON2005, Warsaw (2005) (arXiv:hep-ph/0509116)