Griffiths Introduction to Elementary Particles · Problem 3.21

Symmetries — David J. Griffiths

Physics — QFT · breit-frame

Problem

In reactions of the type A+B→A+C1+C2+⋯A + B \to A + C_1 + C_2 + \cdots (in which particle AA scatters off particle BB, producing C1,C2,…C_1, C_2, \ldots), there is another inertial frame [besides the lab (BB at rest) and the CM (pTOT=0\mathbf{p}_{\text{TOT}} = 0)] which is sometimes useful. It is called the Breit, or ``brick wall,'' frame, and it is the system in which AA recoils with its momentum reversed (pafter=−pbefore\mathbf{p}_{\text{after}} = -\mathbf{p}_{\text{before}}), as though it had bounced off a brick wall. Take the case of elastic scattering (A+B→A+BA + B \to A + B); if particle AA carries energy EE, and scatters at an angle θ\theta, in the CM, what is its energy in the Breit frame? Find the velocity of the Breit frame (magnitude and direction) relative to the CM.

Browse the problem library