Matter/antimmatter annihilation

[Jasapski]

I know that matter and corresponding antimatter particles annihilate giving energy and, conversely a photon can give a pair of particle / antiparticle.

Perhaps this is a candid question, but is it only an empiric finding? or is there other reason? In fact the two particles of the pair have equal mass, different sign for charge... Mass does not annihilate mass and two particles of different charge or different spin do not annihilate either. I see the fact but I do not see the reason. Perhaps this is a basic fat not reducible to other but I would ask your opinion about it.

 

[mfb]

All our physical laws are based on observations. We observe that charge (and other quantum numbers) is conserved. Photons are uncharged. Therefore, only particles with opposite charge (and corresponding other quantum numbers) can annihilate to photons.
In general, all processes which satisfy all conservation laws are possible.

 

[Jasapski]

Thank you mfb. I think you gave me the right clue. Annihilation is the only outcome that satisfy conservation. One could think that other outcomes are possible. I can see that one new particle of mass 2m cannot be created because such a particle does not exist. I am not so sure about a bound, even if transient state.

 

[mfb]

Well, you can have electrons and positrons with higher energy (=> moving relative to each other), and create all sorts of new particles when they react. This is done at electron/positron colliders.

 

[DEvens]

Matter and anti-matter are fundamentally related to each other through
time reversal invariance. An anti-particle is in effect the same as its
corresponding particle going backward in time. If you build a relativistically
invariant theory then you find there are always particle and anti-particle
soulutions. This was pointed out by Dirac before the positron was discovered.

So electrons are time-reversed positrons.

The conversion to energy depends on the fact that there is an interaction
between the two that produces two photons. If you look about 3/4 of the way
down on this page

http://en.wikipedia.org/wiki/Quantum_electrodynamics

you will see Compton scattering with a photon hitting an electron.
But the "same" graph with the outgoing electron moved down and
the incoming photon moved up, represents a positron interacting
with an electron, and producing two photons. Or the other way
over and it's two photons turning into an electron and a positron.

This "simple" symmetry gets a bit more complicated when things like
parity are included. And a bit more when charge conjugation is also.

But basically, the quantum numbers of a particle are reversed under
time reversal. So in order to have time-reversal symmetry you must
have the particle and anti-particle solutions.
Dan

 

[Phenylflux]

During annihilation the positron and electron both emit photons before the actual annihilation in the process of bremsstrahlung as the circle each other. Why is that?

 

[mfb]

I am not sure how to interpret your question, so I answered two interpretations I can think of:

• The annihilation cross-section increases with lower energy. Therefore, most positrons slow down (in matter and if the energy is not too high, this usually happens via Bremsstrahlung) before they annihilate.
• Do not take Feynman graphs literally. They are a tool to visualize calculations, and they can look interesting, but particles are not billard balls shooting around like those graphs could suggest.

 

[Phenylflux]

I remember the eqn for the microscopic cross section for neutron absorption in u235 is corrected inversely for an increase in speed (along with the [u235] and the thermal neutron flux) is the annihilation cross section eqn similar then?

 

[mfb]

They both share the increased cross-section for lower velocities, but the processes are different. The neutron absorption cross-section shows additional features related to resonances.