I just started a class in particle physics this semester, and I really need help on one of our first homework problems. We are supposed to show that while the following decays are allowed: roe(+) --> pi(0)pi(+) roe(-) --> pi(0)pi(-) roe(0) --> pi(+)pi(-) the following decay is not allowed: roe(0) --> pi(0)pi(0) I am pretty sure the reason is that it violates a conservation law, but I don't know which one. We have barely touched on what quantities are conserved in lecture. I can see that charge is conserved in all cases, so no problem there. We are also told that the roe meson is a spin 1 particle, so I think that should play a role. However, the part of quantum mechanics that dealt with angular momentum and spin was a bit treacherous for me. Any help is greatly appreciated.