Breit Frame of reference ( Brick Wall )

Breit Frame of reference ("Brick Wall")

I am having a lot of trouble understanding breit frame of reference.I was trying to solve griffiths problem 3.21 (1st ed.):
Consider elastic scattering- A+B----->A+B If particle A carries energy E, and scatters at an angle theta in the CM,
a) what is it's energy in the breit frame
b)Find the direction and magnitude and direction of breit frame.

I did do the first part by using the fact that in breit frame, (p before)= -(p after) and using the invariant 4 vector dot product of energy-momentum.

I am flummoxed by the b part.Any hints will be greatly appreciated.
Re: Breit Frame of reference ("Brick Wall")

This is clearly a 2-dimensional problem (everything remains in one plane). Let us think about the problem without any calculation. Can you tell something about the projections of A momenta (before and after) along different directions ? Is there a special direction in which the projections are equal ? Is there another direction in which the projections are opposite ? What is the angle between those two special directions ?

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