Describing Massive Theories by Dimensional Reduction

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Hi,

I know that massive field theories are not conformally invariant. However one can incorporate mass by adding an extra dimension, and setting its corresponding momentum to be equal to mass and thereby modifying the constraint equation for spin. The next step as I understand is to perform a unitary operation on the field equation that "introduce factors of [I]i[I] to restore reality". I quote this from Warren Siegel's book - Fields. I don't understand why we need to perform the unitary transformation to get the final field equation. It would be really helpful if someone could answer my question.

Thanks in advance,
C.