1. The problem statement, all variables and given/known data The reaction dd → απ0 (where α is the Helium-4 nucleus and d denotes the deuteron) has never been observed. Why? 2. Relevant equations 3. The attempt at a solution So I checked first the masses to make sure energy/momentum are conserved and found out that mass of 2d is 3751.2 MeV while the decay products have a combined mass of 3862.27 which is greater than the initial state and thus violates conservation of mass. Could this possibly be the case? It just seems simple and we're currently studying topics such as isospin and parity and so I had checked things like charge and baryon number conservation and they seemed alright so I figured I might ask here. Thanks guys!