How Should Spinors Be Applied in Physics Homework Involving Fermions?

lefebvre
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Homework Statement


9oLMYdq.jpg


Homework Equations

The Attempt at a Solution


I've started from writing out the amplitude. Here I know that fermion has definite helicity so I can't sum over spins but I should input explicit forms of spinors. Am I correct? How to do this?
I would be grateful for helping me with this step. I'll manage the rest of the problem.
 
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You insert the expression for a spinor of the appropriate spin. It should be described in your textbook how such a spinor looks.
 
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