N = 4 supersymmetric Yang–Mills theory
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N = 4 supersymmetric Yang–Mills theory is a mathematical and physical model created to study particles through a simple system, similar to string theory, with conformal symmetry. It is a simplified toy theory based on Yang–Mills theory that does not describe the real world, but is useful because it can act as a proving ground for approaches for attacking problems in more complex theories. It describes a universe containing boson fields and fermion fields which are related by 4 supersymmetries (this means that swapping boson, fermion and scalar fields in a certain way leaves the predictions of the theory invariant). It is one of the simplest (because it has no free parameters) and one of the few finite quantum field theories in 4 dimensions. It can be thought of as the most symmetric field theory that does not involve gravity.
where and the indices i,j = 1, ..., 6 and the indices a, b = 1, ..., 4.
The above Lagrangian can be found by beginning with the simpler ten-dimensional Lagrangian
where I and J are now run from 0 through 9 and are the 32 by 32 gamma matrices. followed by adding the term with which is a topological term.
The components of the gauge field for i = 4 to 9 become scalars upon eliminating the extra dimensions. This also gives an interpretation of the SO(6) R-symmetry as rotations in the extra compact dimensions.
By compactification on a T6, all the supercharges are preserved, giving N = 4 in the 4-dimensional theory. The conformal symmetry manifest in this presentation also survives the compactification giving a superconformal field theory.
The coupling constants and g naturally pair together in the form:
This theory is important also in the context of the holographic principle. There is a duality between Type IIB string theory on AdS5 × S5 space (a product of 5-dimensional AdS space with a 5-dimensional sphere) and N = 4 Super Yang–Mills on the 4-dimensional boundary of AdS5. It is the most successful realization of the holographic principle, a speculative idea about quantum gravity originally proposed by Gerard 't Hooft and improved and promoted by Leonard Susskind.
Beisert et al. give a review article demonstrating how in this situation local operators can be expressed via certain states in "spin" chains, but based on a larger Lie superalgebras rather than su(2) for ordinary spin. These are amenable to Bethe ansatz techniques. They also construct an action of the associated Yangian on scattering amplitudes.
- Matt von Hippel. "Earning a PhD by studying a theory that we know is wrong". Ars Technica.
- Luke Wassink (2009). "N = 4 Super Yang–Mills theory". Retrieved 2013-05-22.
- Beisert, Niklas (January 2012). "Review of AdS/CFT Integrability: An Overview". Letters In Mathematical Physics 99: 425.
- Nima Arkani-Hamed; Bourjaily, Jacob L.; Freddy Cachazo; Goncharov, Alexander B.; Alexander Postnikov; Jaroslav Trnka (2012). "Scattering Amplitudes and the Positive Grassmannian". arXiv:1212.5605 [hep-th].