# Why does photon have only two possible eigenvalues of helicity

Photon is a spin-1 particle. Were it massive, its spin projected along some direction would be either 1, -1, or 0. But photons can only be in an eigenstate of $S_z$ with eigenvalue $\pm 1$ (z as the momentum direction). I know this results from the transverse nature of EM waves, but how to derive this from the internal symmetry of photons? I read that the internal spacetime symmetry of massive particles are O(3), and massless particles E(2). But I can't find any references describing how E(2) precludes the existence of photons with helicity 0.

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