S-Channel Process: a+b->c+d Diagrams

[Silviu]

Hello I encountered a problem in which it is specify that there are 2 time-ordered diagram for the s-channel for a certain process a+b->c+d. I understand that there is the one that can be associated with annihilation, but what would be a second one?

Thank you!

 

[nrqed]

Hello I encountered a problem in which it is specify that there are 2 time-ordered diagram for the s-channel for a certain process a+b->c+d. I understand that there is the one that can be associated with annihilation, but what would be a second one?

Thank you!

If you are working with time-ordered diagrams, things are different than with usual Feynman diagrams.
One of the time ordered diagrams is the one you have in mind. For the second, a and b annihilate into a photon (or whatever you are dealing with), this photon travels then the left (it travels "back in time") and then it produces c and d. In other words, in time ordered diagrams, the time ordering of the vertices is crucial, having the vertex "a+b to a photon" before the vertex "photon to c+d" is different from the other ordering. The usual Feynman diagram combines these two into a single expression.