1. The problem statement, all variables and given/known data The problem is: A proton collides with a Helium nucleus. What is the minimum lab kinetic energy needed to produce an anti-proton? 2. Relevant equations 3. The attempt at a solution I suppose that helium nucleus is at rest in the lab frame. I'm wondering about the final particles of this reaction. Originally, I guess the result is a proton, an anti-proton and the helium nucleus, but in this way, the charge is not conserved. So I guess the final particles are two protons , an anti-proton and helium nucleus, but my friend told me that the standard answer in the text is that the final state is with one proton, one anti-proton and one helium nucleus. So, the charge can be not conserved in a reaction? (This means that the interaction is not electromagnetic?) Thanks for all ideas.