1. The problem statement, all variables and given/known data We are the given the rare decay: π+ → e+ + ve Also, we are given mπ = 139.6 MeV/c2, mv ≈ 0, me+ = 0.511 MeV/c2 And we are given that the decay starts with the pion at rest, so I took that to mean that pπ = 0 Solve for the momentum of the positron. 2. Relevant equations Conservation of Momentum Conservation of Energy E2 = (pc)2 + (mc)2 E = mc2 3. The attempt at a solution I started with the fact that since the pion was at rest before decay, pπ = 0 so pe + pv = 0. Then, also using the fact that the pion was at rest before decay I solved for its energy using the formula E = mc2, substituting mπ = 139.6 MeV/c2 getting that Ebefore = 139.6 MeV. Ebefore = Eafter = Ee + Ev (Ee)2 = (pec)2 + (mec2)2 And because the neutrino is massless: Ev = pv*c From before, since Ee + Ev = 139.6 MeV, I solved for Ee to get Ee =139.6 MeV - Ev. Then I substituted pvc for Ev to get the equation: (139.6 MeV - pvc)2 = (pec)2 + (mec2)2 The (pc)2 would cancel each other out, leaving: -279.2pvc = -19487.8 MeV So, pv = 69.799 MeV/c leaving pe = -69.799 MeV/c My question is, is this right, or have I made some glaring mistakes in my method?