What is "spurion"

n. (context physics English) A theoretical particle used in a Lagrangian function to balance its symmetry.

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.

The list of operators found in this way is complete as long as all sources of the breaking are included. The operators in the actual theory are ultimately found by setting the spurious field equal to the constant value of the parameter.