# Matter - anti matter annihilation

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1. Jun 6, 2015

### Stephanus

Dear PF Forum,
Can proton be annihilated by anti neutron?
Can proton be annihilated by positron?
Can neutron be annihilated by positron?
Does every particle has to match its antiparticle to be annihilated, example: Proton must be annihilated by anti-proton, not by anti neutron.
I've heard that there's anti neutrino, too.
Does every particle fundamental also, such as quark, have coresponding anti particle?
Are there particles that have no antiparticle?
I know, should have searched google, but to find this particular answer takes every long time.

2. Jun 6, 2015

### vanhees71

You can only annihilate a particle and its antiparticle. There are conservation laws telling you what's allowed and what not. E.g., in any reaction the electric charge, the baryon number, and the lepton number are conserved. Of course also energy and momentum are conserved.

So you cannot annihilate an proton with a postron (to, say, photons), because then you violate baryon and lepton number conservation.

You can annihilate a proton and a antiprotron to, e.g., two photons, because then you have zero net-baryon and net-lepton number in the initial and the final state. Also energy and momentum conservation can be satisfied, and that's why this processs is allowed and usually, if something is not forbidden by some conservation law, it will happen in nature :-)).

3. Jun 6, 2015

### ChrisVer

Up to now yes, every particle has an antiparticle...

Yes and maybe neutrino-antineutrino are identical particles (if neutrinos are Majorana)

4. Jun 6, 2015

### Staff: Mentor

That process is possible, but extremely unlikely. Annihilation of hadrons usually happens via the strong interaction, and the result are a few pions, both neutral and charged ones.
The neutral pions then decay to two photons each, the charged pions decay to muons and neutrinos.

Proton plus antineutron looks the same, but there is one more positive pion than negative pions.

There are neutrinos and antineutrinos, but their interaction is very weak and they are not charged, so an annihilation to photons is extremely unlikely, and there is not enough energy for other annihilation processes at low neutrino energies.
There are particles that are their own antiparticles, like the photon, the Z boson or the Higgs boson.

5. Jun 6, 2015

### Stephanus

Neutrino and antineutrino are indentical?
What does that mean?
Does that mean neutrino can't be annihilated by antineutrino?
or Neutrino does not have anti particle?

6. Jun 6, 2015

### Staff: Mentor

Maybe. It is not excluded by experiments. We don't know.

Annihilation is practically impossible no matter whether particle and antiparticle are identical or not.

7. Jun 6, 2015

### ChrisVer

It would mean that a neutrino can also annihilate with another neutrino... leaving open windows for neutrinoless double beta decay (2 beta decays that happen simultaneously and the "emitted" neutrino from the one vertex is absorbed by the other vertex, meaning that it can act as both neutrino or antineutrino- poorly explained), and experiments searching for that process. Up to now experiments haven't given results of confirmation and that's why I said "maybe".

http://en.wikipedia.org/wiki/Double_beta_decay#Neutrinoless_double_beta_decay

8. Jun 6, 2015

### ChrisVer

^editted the explanation

9. Jun 6, 2015

### Stephanus

What?? And there are 1089 neutrinos roaming this very universe. I hope they are massless, otherways
$E = mc^2$ is a nightmare.

10. Jun 6, 2015

### ChrisVer

11. Jun 6, 2015

### Staff: Mentor

They are certainly not massless. The precise mass is not known, but it has to be in the range of meV (milli, not Mega).

12. Jun 6, 2015

### Stephanus

Okay..., so neutrino has antiparticle, they somewhat are Majorana particle, http://en.wikipedia.org/wiki/Majorana_fermion,
And they has tiny mass.
Thanks everybody

13. Jun 6, 2015

### Staff: Mentor

MAYBE.
We do not know that.

14. Jun 6, 2015

### Stephanus

Can I add another question here.
CERN are creating anti matter. What particle that they create?
Hydrogen? Helium? Iron? or just elementary particle such has Quark?
What particle do we have if we focus high energy on a single point?
Thanks

15. Jun 6, 2015

### ChrisVer

If you mean creating antimatter in a controlled way: They trap antiprotons and positrons and then try to form antihydrogen atoms (see ALPHA Experiment).
However antimatter can also be produced in the particle collisions together with matter "spontaneously". So when you send two protons to collide (=their constituents collide @ that energies) you get a lot of things coming out. In this large amount of particles coming out and because of the allowing energies, you can have both matter and antimatter . All possible results can appear.

16. Jun 6, 2015

### Staff: Mentor

The high-energetic collisions produce all elementary particles and antiparticles (and also elementary particles where this classification does not make sense).
The only composite particles where it makes sense to call them "antimatter" are antibaryons and antinuclei (made out of those antibaryons). Antiprotons and antineutrons are produced in large quantities, some other antibaryons are quite frequent, but antinuclei made out of 2 (antideuterium) or 3 (antitritium or antihelium-3) antibaryons are very rare. Just a few nuclei of antihelium-4 have been observed so far (2011 at RHIC, not at the LHC), and it is unlikely that heavier antinuclei get produced in existing colliders. No way to get antiiron.