Gauge Fixing
The Proca action is the gauge-fixed version of the Stueckelberg action via the Higgs mechanism. Quantizing the Proca action requires the use of second class constraints.
They are not invariant under the electromagnetic gauge transformations
where f is an arbitrary function, except for when m = 0.
Read more about this topic: Proca Action
Famous quotes containing the word fixing:
“he dreadful darts
With rapid glide along the leaning line;
And, fixing in the wretch his cruel fangs”
—James Thomson (17001748)