Current algebra

Certain commutation relations among the current density operators in quantum field theories define an infinite-dimensional Lie algebra called a current algebra.^[1] Mathematically these are Lie algebras consisting of smooth maps from a manifold into a finite dimensional Lie algebra.^[2]

History

The original current algebra of Gell-Mann of 1964 described weak and electromagnetic currents of the strongly interacting particles, hadrons, leading to the Adler–Weisberger formula and other important physical results. The basic concept, in the era just preceding quantum chromodynamics, was that even without knowing the Lagrangian governing hadron dynamics in detail, exact kinematical information – the local symmetry – could still be encoded in an algebra of currents.^[3]

Example

In a non-Abelian Yang–Mills symmetry, where ρ is the charge density, an example of a current algebra is^[4]

$[\rho ^{a}({\vec {x}}),\rho ^{b}({\vec {y}})]=if_{c}^{{ab}}\delta ({\vec {x}}-{\vec {y}})\rho ^{c}({\vec {x}})$

where f are the structure constants of the Lie algebra. To get meaningful expressions, these must be normal ordered.

Conformal field theory

For the case where space is a one-dimensional circle, current algebras arise naturally as a central extension of the loop algebra, known as Kac-Moody algebras or, more specifically, affine Lie algebras. In this case, the commutator and normal ordering can be given a very precise mathematical definition in terms of integration contours on the complex plane, thus avoiding some of the formal divergence difficulties commonly encountered in quantum field theory.

When the Killing form of the Lie algebra is contracted with the current commutator, one obtain the energy-momentum tensor of a two-dimensional conformal field theory. When this tensor is expanded as a Laurent series, the resulting algebra is called the Virasoro algebra.^[5] This calculation is known as the Sugawara construction.

The general case is formalized as the vertex operator algebra.

See also

• affine Lie algebra
• Virasoro algebra
• Vertex operator algebra
• Chiral model

Notes

1. Goldin 2006
2. Kac, Victor (1983). Infinite Dimensional Lie Algebras. Spring-Science+Business Media, LLC. p. x. ISBN 978-1475713848.
3. Gell-Mann & Ne'eman 1964
4. Treiman, Jackiw & Gross 1972
5. Fuchs, Jurgen (1992), Affine Lie Algebras and Quantum Groups, Cambridge University Press, ISBN 0-521-48412-X

References

• M. Gell-Mann; Y. Ne'eman, eds. (1964). The Eightfold Way. W. A. Benjamin. LCCN 65013009.
• Goldin, G.A. (2006). Françoise, J-P.; Naber, G. L.; Tsun, T. S., eds. Encyclopedia of Mathematical Physics. Current Algebra. ISBN 978-0-12-512666-3 – via ScienceDirect. (subscription required (help)).
• Treiman, S. B.; Jackiw, R.; Gross, D.J. (2015) [1972]. Lectures on current algebra and its applications. Princeton Series in Physics. Princeton, N.J.: Princeton University Press. doi:10.1515/9781400871506. ISBN 978-1-4008-7150-6 – via De Gruyter. (subscription required (help)).