1. The problem statement, all variables and given/known data A pion at rest (mπ = 273me) decays into a muon (mμ = 207me) and an antineutrino (mn ≈ 0). Find (a) the kinetic energy of the muon and (b) the energy of the antineutrino in electron volts. 2. Relevant equations K = (γ-1)mc2 E = γmc2 ER = mc2 E2 = p2c2 + (mc2)2 I didn't use all of these, but I'm assuming a solution requires one or more of them. 3. The attempt at a solution Not really sure how to go about this when the mass of the neutrino is so close to zero. If I assume the total energy after the reaction of the neutrino is essentially zero, then the missing energy goes into the motion of the muon. Kμ ≈ ERπ - ERμ = c2 (mπ - mμ) = c2 (0.511*106eV/c2)(273-207) = 3.37*107 eV = 33.7 MeV The answer in the book is 4.08MeV so either my assumption that Kn ≈ 0 is a bad one, or I've made a mistake somewhere here.