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# Ratio of cross-sections; ratio of transition amplitudes Watch

1. I am trying to do questions (without given solutions) regarding ratios.

They are 5 marks each (of 100 for each paper).

One gives the ratio of the cross-sections of two electron-positron collision processes. It asks to draw the lowest-order Feynman diagrams and discuss how these are used to calculate the cross-sections, and why it is useful to use a ratio R.I'm not sure where to begin! The more urgent question however is about how to estimate the ratio of 'transition amplitudes' for 2 processes (without considering phase-space factors). I am not sure how to do this. The question seems nonsensical to me (I am sure it is fine though) because one process involves an electron and positron combining to give an electron and positron, and the other involves two photons producing two photons. What does that mean though?! Thank you! Edit: the processes for the second part are Bhabha and Delbruck.
2. Would the ratio for Bhabha be e^2 + e^2, for Delbruck e^4 (from the square loop Feynman diagram), so the ratio is 2/e^2? Is this how the ratio works?

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Updated: August 18, 2016
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