How can a particle be its own antiparticle?

[RJ Emery]

Summary:

matter, anti-matter pair

Main Question or Discussion Point

I seek an explanation as to how a particle can be its own anti-particle. I would think the instant such a particle comes into existence, it would self-annihilate.

 

[Orodruin]

Summary:: matter, anti-matter pair

I would think the instant such a particle comes into existence, it would self-annihilate.

Why? There is absolutely no rationale for such an argument.

 

[mfb]

If a particle meets its antiparticle it is possible that the two react. If the reaction products are massless or much lighter (which is again possible but not guaranteed) we typically call this reaction "annihilation". The classical example is electron+positron -> 2 photons.
Such a reaction always needs two particles. If you have a single particle then it might be able to decay. As an example, a Z boson (which is its own antiparticle) will decay quite quickly, typically to a particle+antiparticle pair. A W boson will decay very fast as well, but it cannot be a particle+antiparticle pair as the W boson has an electric charge. A photon (which is its own antiparticle as well) can't decay, it is stable.

 

[Paul Colby]

A photon (which is its own antiparticle as well) can't decay, it is stable.

Can two photons annihilate? I would think not because they don't interact very strongly. But they can interact. Delbruck scattering is an example. I'm confused.

 

[snorkack]

A photon (which is its own antiparticle as well) can't decay, it is stable.

There is the maxim that "if a process does not violate a conservation law, it should happen".
How to pinpoint the conservation law violated by decay of a photon into several?
You can take care of energy conservation (resulting photons combined have the same energy as the initial), momentum conservation (products combined have the same momentum, meaning they travel in the same direction), angular momentum conservation (suitable state of polarization).
Classically it´ s obvious - a plane wave of a given frequency cannot spontaneously change its frequency. But viewing it as a photon subject to conservation laws only, which one specifically forbids it to do such an absurd thing?

 

[Orodruin]

Can two photons annihilate?

Yes. Since an electron-positron pair can annihilate to two photons, two photons (given sufficient invariant mass for the pair) can reverse this process. What the cross section is is another matter.

 

[mfb]

Can two photons annihilate? I would think not because they don't interact very strongly. But they can interact. Delbruck scattering is an example. I'm confused.

They can react (and we have found reactions), but I don't think "annihilation" is a good name for that.

Classically it´ s obvious - a plane wave of a given frequency cannot spontaneously change its frequency. But viewing it as a photon subject to conservation laws only, which one specifically forbids it to do such an absurd thing?

There is some symmetry that tells you the cross section is exactly zero, I forgot the name.

There is also an interesting consistency argument that you can find in more detail with the search function: If a photon can decay, how does its lifetime depend on the energy? It turns out it's impossible to make that decay Lorentz invariant without absurd consequences.

 

[Paul Colby]

They can react (and we have found reactions), but I don't think "annihilation" is a good name for that.

In QED a positron and electron can annihilate to form 2 photons. Just run the reaction diagrams backward in time and I'm good to go. So yeah, looks like annihilation of photons to me.

 

[mfb]

$H\to\gamma\gamma$ is a decay but $\gamma\gamma\to H$ is not. $e+p \to n+\nu$ is electron capture but $n+\nu \to e+p$ is neutrino capture. There is no rule that would give time-reversed processes the same name.

 

[Paul Colby]

There is no rule that would give time-reversed processes the same name.

Then what are the rules? Particle meets antiparticle to produce other particles is the definition of particle annihilation hocked up by Google when queried. Is there a more technically correct one?

 

[mfb]

What I wrote earlier:

If the reaction products are massless or much lighter (which is again possible but not guaranteed) we typically call this reaction "annihilation".

But ultimately it doesn't matter. You can call annihilation what you want as long as it doesn't lead to confusion.