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I have a question regarding the process ##e^+ e^- \rightarrow Z/\gamma \rightarrow f \bar{f}##, where ##f## is a fermion and ##\bar{f}## is an antifermion. I am studying the process to understand LEP measurements.

Supposing of being in ultrarelativistic regime, so helicity and chirality can be seen as the same thing.

Considering the case of the photon, the collision in the CM between a LH electron (spin aligned with its motion) and a RH positron (spin aligned with its motion) gives zero contribution to the matrix element since the spins sum to zero. The photon is a massless spin-1 particle so its spin component can be only ##\pm 1##.

Reading in books I've found that the same holds for the ##Z##. But the ##Z## is massive, so its spin can be zero, isn't it?