Hello guys, Suppose a process 1+2--->3+4 proceeds thru 3 different intermediate particles and has 3 Feynman diagrams. When we compute the probability then what does the cross terms( interference ) mean? and do we have to always compute the cross terms? What I mean is that when you have 3 Feynman diags for a process then, you add the amplitudes and then absolute square? or you absolute square and then add? why and why not? Suppose also that the coupling constants are all different in the 3 Feynman diagrams. Can anybody tell me that are there any conditions when you dont consider those cross terms or interference terms? If you know of any text or article where I can read about this then help me out. THX.