# Are there requirements for Matter and Antimatter to annihilate each other?

#### cosmicweb

Matter and antimatter particles are always produced as a pair and, if they come in contact, annihilate one another, leaving behind pure energy.
Do the pairs have to be those exact pairs to touch and then annihilate? Does matter and antimatter have to be of the same element to annihilate?

What are the chances of antimatter surviving the first second of the big bang?

#### Woody

Good questions. To be honest, I don't know.

What happens when a particle meets it's antimatter counterpart is reasonably clear.
But what happens when an antiproton meets a neutron or a positron meets a muon (for example).

#### cosmicweb

They assume the big bang was pure energy and because of that it created matter and antimatter, correct? They don't know why it wasn't an even annihilation ..what if that pure energy started in a preexisting quantum realm that had dark matter already in it? Dark Matter did something to offset the 100% annihilation.

#### topsquark

Forum Staff
They assume the big bang was pure energy and because of that it created matter and antimatter, correct? They don't know why it wasn't an even annihilation ..what if that pure energy started in a preexisting quantum realm that had dark matter already in it? Dark Matter did something to offset the 100% annihilation.
At this point (and I stress "at this point" as theories about this change rapidly) it is taken that for every one billiion particles of anti-matter from the BB there was one billion and one matter particles. This assumes that the entire Universe is made of matter, a comment that is very hard to test for, to say the least.

There is actually nothing called "pure energy." The energy has to be carried by some particle (or anti-particle) or another.

-Dan

benit13

#### cosmicweb

How would you know? Pure Energy could just be the very first thing that happened ..and then never happened again. I agree that the annihilation probably isn't pure energy ..but what created them might have been.

#### benit13

Do the pairs have to be those exact pairs to touch and then annihilate?
Yes. All of the quantum numbers need to be opposite between the two particles so that the result is a gauge boson.

Does matter and antimatter have to be of the same element to annihilate?
Annihilation is generally described in terms of fundamental particles and elements describe atoms, not fundamental particles, so it doesn't really make sense. The most common annihilation is the $$\displaystyle e^- + e^+ -> \gamma$$, but any particle -anti-particle pair will work.

What are the chances of antimatter surviving the first second of the big bang?
If you mean anti-hydrogen, then I don't know; possibly. Perhaps read some papers on big bang nucleosynthesis? Following baryogenesis, the antimatter seems to have been completely destroyed because it's not observed.
If you mean positrons or anti-neutrinos, then sure.

Can you give specifics about what you're investigating?

topsquark

#### benit13

But what happens when an antiproton meets a neutron or a positron meets a muon (for example).
For the antiproton and neutron, there will be a strong interaction. It might get complicated depending on the particular Feynmann diagram describing the interaction, but no annihilation unless one of the intermediate states involves a proton (e.g. weak interaction causes an up quark to become a down quark in the neutron).

For the positron and muon, there will either be an EM interaction, most likely inelastic scattering, or a weak interaction.

topsquark

#### Farsight

But what happens when an antiproton meets a neutron
They annihilate. Search on proton antineutron annihilation.

The thing about all of this that people don't appreciate is that postronium is a short-lived "exotic atom". It's comprised of both matter and antimatter. And it's sometimes described as "light hydrogen".

#### topsquark

Forum Staff
They annihilate. Search on proton antineutron annihilation.

The thing about all of this that people don't appreciate is that postronium is a short-lived "exotic atom". It's comprised of both matter and antimatter. And it's sometimes described as "light hydrogen".
Just to stress the point:
$$\displaystyle p^+ \bar{n} == (uud)(\bar{u} \bar{d} \bar{d} ) \rightarrow (u \bar{u} ) (d \bar{d} ) (u \bar{d}) == \gamma \gamma \pi ^+$$

As the proton and anti-neutron are not particle/anti-particle pairs there won't be a complete cancellation.

-Dan

#### Farsight

As the proton and anti-neutron are not particle/anti-particle pairs there won't be a complete cancellation.
Noted Dan. I think what's interesting is that the charged pion decays into a muon and a neutrino, then the muon decays into an electron and more neutrinos. So all you've got left in your hand is an electron. Everything else has departed at the speed of light. Then if you had a positron handy, you could annihilate that electron. Then your proton and your antineutron have been rendered down to photons and neutrinos. It's as if these are the lowest common denominators of matter.

Anyway, the thing I was getting at with the positronium is that baryon asymmetry is matched by lepton asymmetry, and that there’s an ambiguity when it comes to matter and antimatter. Let's forget about neutrons for a minute, and focus on electrons and protons and their antiparticles. If we have four particles a b c d we could label two of them matter and two of them antimatter. Let’s say a and b are matter and c and d are antimatter. We can then pair up those four particles in six different various ways: ab ac ad bc bd and cd. If we could somehow stick the two matter particles a and b together we could reasonably call the result “matter”. There’s only one way out of six that we could do this: ab. If we could somehow stick the two antimatter particles together we could reasonably call the result “antimatter”. There’s only one way out of six that we can do this: cd. However the other four combinations ac ad bc bd consist of both matter and antimatter. So there’s twice as many ways to make an exotic atom as there are to make an atom of matter or antimatter. The mystery of the missing antimatter is that there’s more particles than antiparticles in the universe. But if we took a tip from positronium and said hydrogen was an exotic atom too, we’d have the same number of particles and antiparticles in our universe. Like this: