If a particle decays via A →B + C, and A had some initial non-zero momentum, is it possible for either B or C to be stationary? I can't seem to find a restriction on this from energy conservation or momentum conservation. From energy conservation, the stationary particle B still contributes energy from its rest mass, so C does not need to have the same mass as A. From momentum conservation, C can carry off all the momentum of A, leaving B free to be stationary. I've never heard of this kind of thing occurring in Nature, and that makes me wonder if it is actually possible for a decay product to be stationary. Thanks a lot.