Collision theory

The BF model is a topological field theory, which when quantized, becomes a topological quantum field theory. BF stands for background field. B and F, as can be seen below, are also the variables appearing in the Lagrangian of the theory, which is helpful as a mnemonic device.

We have a 4-dimensional differentiable manifold M, a gauge group G, which has as "dynamical" fields a two-form B taking values in the adjoint representation of G, and a connection form A for G.

The action is given by

$S=\int _{M}K[\mathbf {B} \wedge \mathbf {F} ]$

where K is an invariant nondegenerate bilinear form over ${\mathfrak {g}}$ (if G is semisimple, the Killing form will do) and F is the curvature form

$\mathbf {F} \equiv d\mathbf {A} +\mathbf {A} \wedge \mathbf {A}$

This action is diffeomorphically invariant and gauge invariant. Its Euler-Lagrange equations are

$\mathbf {F} =0$ (no curvature)

and

$d_{\mathbf {A} }B=0$ (the covariant exterior derivative of B is zero).

In fact, it is always possible to gauge away any local degrees of freedom, which is why it is called a topological field theory.

However, if M is topologically nontrivial, A and B can have nontrivial solutions globally.

See also

• Spin foam
• Background field method

External links

• http://math.ucr.edu/home/baez/qg-fall2000/qg2.2.html