A Λ0 particle of rest mass 1115.7 MeV and kinetic energy 172.6 MeV decays into proton of mass 938.8 MeV/c^2 and momentum 428.8 MeV/c and a "mystery" particle - calculate mass of mystery particle and hence deduce what type of particle it is.
I have done the calculation and get a total energy of 215.8 MeV and momentum of 129.05 MeV/c for the "mystery" particle. Hence the rest mass I get is 172.96 MeV/c^2. I know the particle must be a negative pion rest mass 140 MeV/c^2. What have I done wrong? How can I get to the correct answer? thanks