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In 90% of the papers I've read about diferent ways to achieve generalizations of the Proca action I've found there's a common condition that has to be satisfied, i.e: The number of degrees of freedom allowed to be propagated by the theory has to be three at most (two if the fields are massless).

I wonder... why is so?, two hypothesis I bare in my mind after discussions with my thesis director were:

1. Proca propagates three degrees of freedom hence, it would be unphysical to pretend to generalize this action at our energy levels including extra degrees of freedom that hasn't been observed in experiments.

2. There could be a fundamental reason related to the representations of the Poincaré group which, as far as I've seen (not much), tell you that two degrees of freedom (2 out of the 4 componets of the vector) are redundant if the vector field is massless and 1 more degree of freedom arises if the mass is non-zero.

The second idea follows from this slides: http://bit.do/UCslides, and this PF post: http://bit.do/PFpost.

Which idea is the most accurate for you guys?

and where could I find more info about it?