Symmetric relativistic equal-mass scattering angle
= Symmetric relativistic equal-mass scattering angle
A particle of <kinetic energy> $\alpha mc^2$ hitting an equal mass at rest, with equal and opposite outgoing angles $\theta$, obeys $\tan\theta=\sqrt{2/(\alpha+2)}$. Conservation of total energy gives equal final energies, and longitudinal <momentum conservation> fixes the angle.