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*For a closed fermionic loop, the Feynman rule is to start at an arbitrary vertex or propagator, follow the line until we get back to the starting point, multiply all the vertices and the propagators in the order of the line, then take the trace of the matrix product. In addition, we include a negative sign for every closed fermionic loop.*

For an open fermionic line, we must trace from the head of the line to the tail. In other words, we must start by writing down the polarization spinor for the line with outgoing charge, ..., and finally the polarization spinor for the line with the incoming charge.

Is there also a rule for closed fermionic loop that we must trace the loop

*in the direction opposite to the direction of the charge flow*?