Gauge invariances in the PROCA model

We show that the Abelian Proca model, which is gauge noninvariant with second class constraints can be converted into gauge theories with first class constraints. The method used, which we call gauge unfixing, employs a projection operator defined in the original phase space. This operator can be constructed in more than one way and so we get more than one gauge theory. Two such gauge theories are the Stückelberg theory and the theory of Maxwell field interacting with an antisymmetric tensor field. We also show that the application of the projection operator does not affect the Lorentz invariance of this model. © World Scientific Publishing Company