1. The problem statement, all variables and given/known data A K meson (an elementary particle with approximately 500 Mev rest mass) traveling through the laboratory breaks up into two Pi mesons (elementary particles with 140 Mev rest energies). One of the Pi mesons is left at rest. What is the total energy of the remaining Pi meson? 2. Relevant equations E=mc^2 KE=(γ-1)mc^2 3. The attempt at a solution My first step was trying to use energy conservation E(initial)=E(final) The K meson is traveling so the total E at the start is E(rest mass of K)+KE(K meson) E(initial)=500 Mev+(γ-1)mc^2 In the end, the K meson breaks into 2 Pi mesons, each with 140 Mev rest mass. One stops and the other keeps on moving. So the total E at the end is E(rest mass Pi)+E(rest mass 2nd Pi)+(γ-1)mc^2 E(final)=140 Mev+140Mev+(γ-1)mc^2 E(initial)=E(final) 500 Mev+(γ-1)mc^2=140 Mev+140Mev+(γ-1)mc^2 But now I don't know what to do next. I don't have a value for gamma.