Questions about the Force Carriers

[Moriarty]

I've been working recently on my understanding of the Standard Model. I was fine (more or less) until I got to the force carriers, which is where my understanding seems to really break down.

 

My questions:

 

1. How can a particle be its own anti-particle, eg, Z particle, photons, gluons and gravitons? What do you get if two photons annihilate with one another...another pair of photons?

 

2. Is there any experimental data short of direct observation to support the existence and modeling of the graviton, or does it simply serve as a placeholder in an attempt to complete the standard model until better information comes along?

 

Thanks in advance. I'm certain to have more questions later.

[swansont]

 

1. How can a particle be its own anti-particle, eg, Z particle, photons, gluons and gravitons? What do you get if two photons annihilate with one another...another pair of photons?

 

The properties that differentiate particles from antiparticles (charge, parity) do not allow us to distinguish the, If the photons annihilated, the new photons would be identical. You can't tell the difference between them.

[Moriarty]

Thanks for the reply.

 

If the photons annihilated, the new photons would be identical. You can't tell the difference between them.

 

I'm with you so far. I don't yet understand the why, but I'll get there.

 

The properties that differentiate particles from antiparticles (charge, parity) do not allow us to distinguish the,

 

If you're saying there is no measurable difference between a photon and it's anti-particle (itself?), then I follow. However...why call it an anti-particle? Do photons actively annihilate one another like electrons/ positrons? If not, how is the interaction different?

 

My real confusion here is in figuring out why some of these force carriers are considered their own anti-particles. Hope my questions make sense.

 

Did you intentionally skip my graviton question?

 

Thanks again.

[cameron marical]

2. Is there any experimental data short of direct observation to support the existence and modeling of the graviton, or does it simply serve as a placeholder in an attempt to complete the standard model until better information comes along?

 

I think that it is more of a theoreticall particle than a proven particle. Although, many theorys include it, I dont think that there is any tests to prove its existence.

 

{I dont know the answer to your first question, sorry. Im standing by awaiting the answer myself.}

[swansont]

If you're saying there is no measurable difference between a photon and it's anti-particle (itself?), then I follow. However...why call it an anti-particle? Do photons actively annihilate one another like electrons/ positrons? If not, how is the interaction different?

 

My real confusion here is in figuring out why some of these force carriers are considered their own anti-particles. Hope my questions make sense.

 

There would be no way to tell the difference. There are no characteristics that are different which would allow you to distinguish between them.

 

Did you intentionally skip my graviton question?

 

Yes. I'm not a particle physicist, and can't give you a definitive answer, though I'm pretty sure the graviton is not part of the standard model.

[Moriarty]

Thanks for your insights, guys. Appreciate the time.

[proton]

1. How can a particle be its own anti-particle, eg, Z particle, photons, gluons and gravitons? What do you get if two photons annihilate with one another...another pair of photons?

By definition - the anti-particle of a particle is the particle which as the same mass but opposite charge and magnetic moment. In those situations when the charge and magnetic moment is conjugated (made opposite) and it results in the same value then the resulting particle is the same as the original particle.

 

The mass, charge and magnetic moment of the photon is (0, 0, 0). Therefore its anti-paricle has the properties (0, 0, 0). This is just a photon. However when a particle is identical to its anti-particle they do not annihilate each other.

[Moriarty]

Ahh, now the Okie gets it. Many thanks.

[proton]

Ahh, now the Okie gets it. Many thanks.

You're most welcome.

 

May I make a suggestion? When you want to ask another question about antiparticles/antimatter I recommend that you post the question under the Modern and Theoretical Physics section. Typically textbooks on quantum mechanics don't cover antiparticles. That's usually covered in texts on Modern Physics and Particle Physics. Particle physicists who frequent this forum may monitor those sections more frequently than this one and may more readily get to your question than otherwise.

[timo]

However when a particle is identical to its anti-particle they do not annihilate each other.

Why do you think that was the case?

[swansont]

You're most welcome.

 

May I make a suggestion? When you want to ask another question about antiparticles/antimatter I recommend that you post the question under the Modern and Theoretical Physics section. Typically textbooks on quantum mechanics don't cover antiparticles. That's usually covered in texts on Modern Physics and Particle Physics. Particle physicists who frequent this forum may monitor those sections more frequently than this one and may more readily get to your question than otherwise.

 

That will probably make little difference in the answers, though, and we frown on posting the same question in multiple areas of the board. I don't think the physicists who post here limit themselves that strictly. I suspect many look at the "new posts" and go from there.

 

If need be, a post can be moved to a more appropriate area.

[cameron marical]

There would be no way to tell the difference. There are no characteristics that are different which would allow you to distinguish between them.

 

Why would you not be able to use something like a magnetic feild wich has either charge {positive or negative}, and see if the particle is attracted or reppeled?

[proton]

That will probably make little difference in the answers,..

I agree. I mentioned it only because if one were to classify antiparticles under quantum mechanics or particle physics then it would be apopropriately listed under particle physics. If someone was curious about antiparticles and wanted to search the forum on previous posts then they should start searching under the Modern and Theoretical Physics section to begin with. However, speaking for myself, I frequent those sections in which have some exertise more than I frequent other sections. For example; I look at the relativity section quite often because that's where my education and experience lies and for that reason that is where I can be of most use. I look at the Modern and Theoretical Physics section much less often for similar reasons. I assumed that others are the same way to at least some extent. Of course this may be quite wrong, I don't know. I'm merely going by my personal experience. For that reason it was why I said it was merely a suggestion. After all, what is the purpose of having different sections if not for similar reasons?

 

It would be a waste of time and energy to put any thought into that suggestion beyond what I said. It's not as if it was very important. In fact it has more to do with where people should look in the physics literature for the subject than anything else.

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Merged post follows:

Consecutive posts merged

Why do you think that was the case?

Because photons don't annihilate themselves.

Edited June 24, 2009 by proton
Consecutive posts merged.

[Severian]

Because photons don't annihilate themselves.

 

Let's leave photons aside for a minute. What about a gluon? Gluons self interact, so two gluons can 'combine' into one gluon. Would you count that as an annihilation?

 

Once you have answered that, speculate on whether or not you think the gluon is its own antiparticle.

 

Edit: rereading my post, it sounds a little patronizing. Apologies, but I think it is a constructive exercise.

[timo]

However when a particle is identical to its anti-particle they do not annihilate each other because photons don't annihilate themselves.

That is quite an extrapolation to say that one pair of particles does not annihilate (whatever "annihilation" exactly shall be) therefore no other pair does either. Z-bosons do annihilate. Depending on whether by "annihilation" you mean that the reaction happens even when there is little to no kinetic energy involved or if you just mean any reaction that changes the particle types then photons either do not annihilate because of conservation of energy or they probably do. I see no reason why electron+positron -> 2 photon should not work the other way round given sufficiently high-energy photons (at least in principle).

[swansont]

Why would you not be able to use something like a magnetic feild wich has either charge {positive or negative}, and see if the particle is attracted or reppeled?

 

You could do this test, but we've already established that the particles under consideration have no charge.

[Severian]

Since everyone ignored my little exercise, the point I was getting at was that gluons are not their own antiparticle (although it is often wrongly stated that they are).

 

Can anyone tell me why? (Yes, I know I am being patronizing again - I can't help it.)

[Baby Astronaut]

Since everyone ignored my little exercise, the point I was getting at was that gluons are not their own antiparticle (although it is often wrongly stated that they are).

 

Can anyone tell me why? (Yes, I know I am being patronizing again - I can't help it.)

I'll bite.

 

A gluon can't be its own antiparticle because the glue holds it firmly in place.

 

No, why?

[swansont]

Since only BA has chimed in, I'll guess (it's not like I know much about QCD) that it's because they have color charge.

[Severian]

Actually, after thinking about this a little more, I think I am talking rubbish. I have convinced myself that they are their own antiparticle.

 

My argument for why they aren't was exactly as Swansont says - they have a color charge, so if you conjugate you end up with the opposite color charge, and can tell them apart.

 

However, I got to thinking about how they are defined in the underlying QFT, and there, your creation and annihilation operators in the gluon field have coefficients which are conjugates of one another (exactly like the photon) which tells me that they are their own antiparticle after all.

 

So what is happening with the color charge? It is slightly more complicated than saying that a gluon is, say, [math]R \bar B[/math] so its conjugate is [math]B \bar R[/math] (where R and B are red and blue, remembering that gluons carry two colors since they are in the adjoint representation) because the color combinations are set by the Gell-Mann matrices.

 

There are 8 Gell-Mann matrices - one for each gluon - and they tell us how the independent gluons are composed. So gluon 1 is set by [math]\lambda_1[/math] which tells us that it is [math]R \bar B + B \bar R[/math]. You can see that under conjugation this does map onto itself. Similarly gluon 2 is set by [math]\lambda_2[/math] which tells us that it is [math]-i (R \bar B - B \bar R)[/math]. Conjugating give the same thing back again (the i takes care of the sign).

 

All the Gell-Mann matrices are like this (they are Hermitian), so each gluon is a combination of color charges such that they map onto themselves under conjugation. Therefore the gluons are indeed their own antiparticles.

 

Sorry for misleading you (however temporarily).