Current algebra

Certain commutation relations among the current density operators in quantum field theories define an infinite-dimensional Lie algebra called a current algebra.

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.

Example

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

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

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. 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