1. The problem statement, all variables and given/known data Ok so im given the rest masses of D meson (1.86 GeV/c^2), pi meson (0.13 Gev/c^2) and K meson (0.5 Gev/c^2) and told that a D meson at rest decays into a K meson and pi meson. Told to calculate the energy and velocity of the pi meson.. Part (b) says the pi meson subsequently decays into a pair of photons. I'm asked to find the maximum and minimum energies of each photon in the D meson rest frame.. 2. Relevant equations 3. The attempt at a solution Ok so I think I did part (a) okay..found the energy of the pi meson to be 0.869 Gev/c^2...is this right? I'm just not sure how to approach part (b).. What does it mean by maximum and minimum energies of each photon? Won't the photons just move off in opposite directions with non equal momenta (depending on initial velocity of pi meson)?