A pion of rest mass decays at rest into a muon of rest mass and a neutrino of zero rest mass. What is the speed of the muon?
In the pion rest frame , the muon moves in the -direction. A moving observer, in a frame with axes parallel to those in the pion rest frame, wishes to take measurements of the decay along the -axis, and notes that the pion has speed with respect to the -axis. Write down the four-dimensional Lorentz transformation relating to and determine the momentum of the muon in . Hence show that in the direction of motion of the muon makes an angle with respect to the -axis, where