Okay here's something I can't answer for myself. We all know what happens when matter and antimatter collide, right? They annihilate each other, leaving behind no mass, only gamma ray energy. Question is, what happens if a neutron collides with an anti-electron (positron)? The positron has much, much SMALLER mass than the neutron. So does the neutron get a little piece of it blown away into energy, then we just have this neutron with a crater in it moving around? But then the neutron wouldn't be a fundamental particle, would it? Hmmm... [EDIT] i realize now that electrons and anti-electrons are NOT made up of 3 quarks like old theories thought. They are fundamental. So let me rephrase the question. What happens when 2 fundamental particles of different mass, and one is normal matter and the other is antimatter, collide? For the sake of concreteness i nominate a muon colliding with an anti-electron.