1. The problem statement, all variables and given/known data A ##\pi^0## at rest decays into two photons, one travelling in the +z direction and the other in the –z direction. What is the angle between the photons in a reference frame moving at speed v=0.99c along the x-axis? 2. Relevant equations 3. The attempt at a solution I'm not sure if I understood the problem correctly but I have made a diagram of the situation below. Since the two decay products have an equal mass (zero) their velocity is c, which is greater than that of the parent neutral π meson. http://imageshack.us/scaled/landing/407/pionp.jpg [Broken] I think the problem is asking for the angle which I've labled as θ in my diagram. So how can I work out this angle? Is the question asking for the minimum/maximum decay angle, or a definite angle? (I'm not sure what principles or equations I have to consider). Any help is greatly appreciated.