I'm studying the $e^{+}e^{-}\rightarrow f^{+} f^{-}$ scattering, where $f$ is a generic fermion ($f^{\pm}\not=e^{\pm}$). I know the process is described by the $s$-channel, but why the $t$-channel or the $u$-channel can't describe it? How can I understand which channel to use to describe the process?
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