Spurion

In theoretical physics, a spurion is a fictitious, auxiliary field in a quantum field theory that can be used to parameterize any symmetry breaking and to determine all operators invariant under the symmetry.

The procedure begins with finding a parameter that measures the amount of symmetry breaking. This parameter is promoted to a field, i.e. to a function of the spacetime coordinates. With this new fictitious field, operators that are invariant under the symmetry may be found by the usual group-theoretical considerations.