Here is a beta decay related question: Masses are from Particle Data Group (http://pdg.lbl.gov/2008/listings/contents_listings.html) Neutron has udd quarks. Mass is approximately 2.5, 5, 5 MeV. Total 12.5 MeV Products are: 1. Proton. uud quarks. Mass approx 2.5, 2.5, 5 MeV 2. Electron. mass about 0.5 Mev 3. Neutrino, mass trivial. Neutron's quark mass of 12.5 MeV gives about 10.5 MeV. Or to look at it differently, down quark converts to up quark and electron. 5 MeV of the down quark gives 3 MeV (2.5 for u quark and .5 for e-) Why the difference? --------- Clarification of my question: My question or puzzle can be more specifically stated as "where is the missing mass?" In a beta decay, (where a down quark decays to an up quark, electron and neutrino) we have an approximate 5.0 MeV particle producing particles of mass 2.55 and 0.511. There is a loss of about 2 MeV, based on the accepted masses of the particles. Where did the lost energy go?