# How can I determine which Feynman diagram picks up a minus sign?

I'm reading David Tong's QFT notes, and am having a question about these diagrams: When computing the matrix element, the second diagram picks up a minus sign 'due to statistics'. My question is does it matter which diagram has the minus sign, and how can I determine which diagram is the $$t$$ channel and which is the $$u$$ channel.

It seems like the $$u$$ channel is always drawn with crossed legs, but I think we can always treat the $$u$$ channel as $$t$$ channel and then assign the minus sign to $$t$$ instead.

• Link? Which page? Commented Dec 12, 2022 at 19:06

• @IGY probability amplitude is not measurable, so you will have to square eventually. In some interpherence experiments the phase may matter (relative to another process) - but I doubt that this is what you are dealing with. Also, the final labels are likely dummy indices, to be summed over in the final cross-section formula - so which diagram has phase is the matter of how you label them and how you draw them (which is Hartree and which is Fock.) In QM $\psi(x)$ and $-\psi(x)$ are the same state. Commented Dec 12, 2022 at 17:29