# Matter and anitmatter

1. Nov 10, 2007

### spideyinspace

i have a doubt..what would happen when matter and antimatter meets..e.g what would be the result of neutrino + anti neutrino...would they go off from our universe

2. Nov 10, 2007

### cristo

Staff Emeritus
No, they would not "go off from our universe." When a particle and an antiparticle meet, they annihilate, and produce two photons.

3. Nov 10, 2007

### spideyinspace

is this is the case with all matter and antimatter..does all matter+antimatter gives two photons?then what is the probabalility that photon becoming which matter and which antimatter?

4. Nov 10, 2007

### mathman

Please clarify this sentence!! Photons are neither matter nor antimatter - if that's what you are asking.

5. Nov 10, 2007

### spideyinspace

what i mean is,electron and positron gives photons,neutrino and anti neutrino also gives photons,so if there is chance for photons to become again matter and antimatter, what is the probability that it would become electron and positron,neutrino and anti neutrino?...

6. Nov 11, 2007

### mathman

It depends on a lot of conditions: The energy of the photon for starters, the environment.

I am most familiar with the electron-positron case. Here the minimum photon energy is 1.022 Mev (the rest mass of the the positron + electron) - gamma rays. Because of momentum consideration, it won't happen by itself, but only with interaction involving a nucleus (for example) - with probability a function of at. wt. of nuclide as well as the energy of the photon. Also for very high concentrations of photons (like just after the big bang), two photons could collide to produce an electron-positron pair.

For other pairs, such as neutrino-antineutrino or proton-antiproton, the same principles apply, with different energy requirements and different probabilities.

7. Nov 11, 2007

### blechman

Neutrinos are electrically neutral, and photons only couple to charge, so how exactly does neutrino-antineutrino annihilate to photons?!

The only thing a neutrino-antineutrino pair can annihliate to are Z-bosons, which are too heavy to be produced explicitly, so they must be virtual, and when these (highly virtual) Z bosons decay, they can go to lots of things (quark-antiquark/lepton-antilepton/etc).

So I think the correct answer is that when a particle and its antiparticle meet, they annihilate into energy, which could come in many forms.

8. Nov 11, 2007

### spideyinspace

so you say neutrinos and anti neutrinos will not give photons...is there any any proof or website related to this..

9. Nov 11, 2007

### Parlyne

Through loops, or course. For example, the pair could produce a virtual e/W loop which could radiate the photons.

The $\nu / \overline{\nu}$ pair could also directly annihilate to a $W^+/W^-$ pair through a virtual electron. Either this or annihilation to a Z (or multiple Zs) would require extraordinarily energetic neutrinos. And, even this, the Ws or Z would quickly decay. So, the observed processes will be those that use these processes to annihilate the neutrinos and, then, have the heavy bosons either decay or only exist as virtual particles in the first place.

10. Nov 11, 2007

### Parlyne

In fact, they can annihilate to photons. For lower energy neutrinos, this is, in fact, the only thing the could annihilate to. However, any interaction that could do this is highly suppressed, first by the fact that neutrinos only directly interact through the weak force, which is very weak, and second by the fact that this process can only occur at second order in the perturbation expansion.

11. Nov 12, 2007

### blechman

OK, neutrinos can annihliate into photons through EW-loops, this is true. But it's subdominant, and not very likely. In fact, probably the most likely thing that would happen if a neutrino met its antineutrino in a bar is that they would completely ignore each other! The elastic scattering should be much larger than the annihilation channel, since it can happen at tree level through only one (virtual) Z-boson exchange, rather than requiring two or more virtual EQ gauge bosons (and possibly also a loop).

12. Nov 12, 2007

### quantumfireball

the electron positron have an amplitude of just scattreing of each other by
exchanging momenta,
its not compulsory that they have to anhillate

13. Nov 12, 2007

### Parlyne

Certainly true. But, if we're talking specifically about annihilation processes, you need to get to somewhat significant energies before any decay channel other than photons opens up in the first place. So, to that extent, it's still the dominant annihilation process. It's simply far less likely than elastic scattering.

14. Nov 13, 2007

### blechman

I'll agree with that.

15. Nov 14, 2007

### spideyinspace

Thank you for all replies..
shall we conclude that
lower energy neutrinos and antineutrinos can annihilate to photons..
higher energy neutrinos and antineutrinos annihilate to quark-antiquark or lepton-antilepton..if this is wrong can one give a conclusion for this topic(neutrino+antineutrino)...

16. Nov 14, 2007

### malawi_glenn

A general rule, the higher energy you have in the CM of two colliding anti particles, the more options for the final decay state. The more energy available: the more particles you can reach.