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I'm wondering if a photon's helicity can change in QED. For example in Compton scattering could the exiting photon have a different helicity than the incoming photon?

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Yes, it can change since what is conserved is the total angular momentum of photon + electron. So as long as the electron carries out an angular momentum such that it compensates the change of the helicity, it is allowed.

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  • $\begingroup$ Indeed. In lower energies, cf the Klein-Nishina formula, you have helicity violation induced by the mass of the electron, so helicity is not a good number--even the helicity of the electron can flip, despite the vectorlike interaction preserving its chirality. $\endgroup$ – Cosmas Zachos Aug 29 '17 at 14:58

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