# Antimatter Question

I am trying to improve my understanding of matter-antimatter annihilation. Obviously, we know that when an electron and a positron meet we have annihilation and photons are produced. However, I am confused over what would happen if a positron met a neutron. Is a proton produced with photons? And what happens if an electron meets an anti-neutron, is an anti-proton produced with photons?

I guess I am simply trying to understand how antimatter reacts to different kinds of matter.

Is there any situation when antimatter wouldn't interact with matter?

• Wikipedia: “Annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles.” [Emphasis mine.] Neither an anti-neutron, nor the antiquarks and gluons inside it, are the antiparticle of an electron. Annihilation is not between random matter and antimatter but between particles and their specific antiparticles. Commented Sep 25, 2022 at 2:59
• So they simply wouldn't interact? And what about extending the argument. If an up quark meets and anti-charmed/top. Simply no interaction? Commented Sep 25, 2022 at 3:04
• Since you asked about a positron and a neutron, I should have said “Neither a neutron, nor the quarks and gluons inside it, are the antiparticle of a positron.” Commented Sep 25, 2022 at 3:09
• Not annihilating is not the same as not interacting. For example, a positron is charged and a neutron has a magnetic moment, so they should interact electromagnetically. They can also interact by the weak interaction, and gravitationally. Commented Sep 25, 2022 at 3:10
• There are no free quarks/antiquarks to do such an experiment. Antiups may meet a top quark only within complicated interactions with many particles output . Commented Sep 25, 2022 at 4:28

Consider Beta decay. The lead Feynman diagram is so.

In a neutron a $$d$$ quark changes to a $$u$$ by emitting a $$W^-$$ Boson. This then decays up into an electron-anti neutrino and an electron.

Now the thing about Feynman diagrams is you can (usually) move a "leg" from up to down by changing the particle to an anti-particle. And you can (again, usually) change all the particles in a diagram to their anti particles and get a still-valid diagram. However, not all such diagrams are equally easy to observe.

This is electron capture. Note that there are two versions. One where the intermediate is a $$W^-$$ (going "backwards" in time) and the other with a $$W^+$$.

But compare to the Beta decay diagram. All three are really the same diagram just with various particles flipped up-to-down and particle-to-anti.

Electron capture is most usually observed in certain types of atom where it is energetically favorable. An electron from one of the atom's orbitals is captures by on of the protons in the nucleus. One proton is replaced by one neutron and a neutrino is emitted.

Positron capture is possible by starting with the Beta decay diagram and flipping the electron down. Instead of an electron leaving it is a positron entering. It is not as easy to observe as electron capture. A positron will tend to find an electron before it finds a nucleus.