particle physics: energy conservation

[kylie14]

Hi,
I've managed to get extremely confused; I feel like I'm getting told different things! I hope someone can just clarify this for me.

If you have a reaction, say for example:
p pion+ --> p p
(where p is a proton) is it true that the rest mass afterwards must be less than the rest mass before or energy conservation is violated?

[Phiphy]

No. The rest mass afterwards only need to be less than the total energy of the initial particles. You can accelerate very light particles, eg., electrons, to get large kinetic energy and produce heavy particles in the end.

[RedX]

You're thinking about 1 particle decaying into a product of particles. In that case, the rest mass of the decay products need to add to be less than the rest mass of the initial particle.

However, as already mentioned, when you have two particles colliding, they can have extra energy beyond their rest mass in the form of a relative momenta.

Also, the particular example you cited violates baryon number conservation.