You are Here: Home >< Physics

# Ratio of cross-sections; ratio of transition amplitudes

Announcements Posted on
Would YOU be put off a uni with a high crime rate? First 50 to have their say get a £5 Amazon voucher! 27-10-2016
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?

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
your full birthday is required
1. Oops, you need to agree to our Ts&Cs to register

Updated: August 18, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Halloween costume competition

Share your boo-tiful pics

### How to cope with a heavy workload

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.