1. The problem statement, all variables and given/known data In the rare decay ∏+ → e+ + ve , what is the momentum of the positron (e+)? Assume the ∏+ decays from rest. (m∏+ = 139.6 MeV/c^2, mv ≈ 0, me = 0.511 MeV/c^2) 2. Relevant equations Conservation of Energy: E∏ = Ee + Ev Conservation of momentum: p∏ = pe + pv 0 = pe + pv pe = -pv Invariant mass: E^2 = (pc)^2 + (mc^2)^2 3. The attempt at a solution My first step was to ensure momentum was conserved, stating that once the pion decayed, the positron and neutrino went off in opposite directions. This yields the equation pe = -pv. Next I went about finding the rest energy of the pion, which following the equation, E∏ = m∏c^2. Using this I found the rest energy of the pion to simply be 139.6 MeV. After that I began to use the formula, E^2 = (pc)^2 + (mc^2)^2. I thought that E in this case is the energy of the system, 139.6 MeV, m is the rest mass of the positron, .511MeV/c^2, and p is the momentum of the system that I am asked to solve for. Solving that equation I found p to be 139.599 MeV/c. This is where I am confused/unsure and could use some help if possible. Is that momentum the momentum of the entire system or of the positron after the decay? Also how do I factor in the neutrino considering it is massless? I know the equation E = pc can be used to solve for the energy of a massless particle, however I am not sure if that needs to be used for this problem. This is my first post here, and I am new to forums in general so I am sorry if I forgot to follow a certain protocol of these forums! Thank you for any help that you may be able to give me!