Apologies, this is going to be a bit disjointed, I dont want to write the full question down as I dont want anyone to give me a solution as its an assignment question. 1. am i correct in assuming when a pion decays at rest its energy will be given by: E^2 = p^2 c^2 + m^2 c^4 which goes to: E^2 = m^2 c^4 because I get something ridiculous like 1394J. ive cancelled the c^2 of the pion mass with the c^4 in the energy eqn. 2. to find the energy of the photons i use conservation of energy: E(pion) = E(photon1) + E(photon2) E(photon1) = GAMMA * m c^2 i) what do i use for the mass of a photon? ii) how do i calculate gamma when no velocity of the photons (or the pion) is given? thanks in advance for any suggestions.