# Massless Anti-Particles?

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1. Jun 20, 2015

### referframe

Are massless particles always their own anti-particle? Thanks in advance.

2. Jun 20, 2015

### Orodruin

Staff Emeritus
No, this is not a necessity.

3. Jun 20, 2015

### referframe

So, the Standard Model allows for the hypothetical existence of a particle that has zero mass and is not equal to it's own anti-particle?

4. Jun 20, 2015

### rootone

Gluons are massless, and come in a variety of flavours which can interact, I'm not sure about the concept of an antigluon.

5. Jun 20, 2015

### Orodruin

Staff Emeritus
The standard model is a particular model with a well defined particle content. It does contain a massless particle which is not its own anti-particle in the massless neutrino. It also contains gluons of different sorts which have different colour charges and differently colour charged particles may or may not be considered as different. In addition, all standard model particles are massless before spontaneous symmetry breaking.

6. Jun 20, 2015

### mathman

Neutrinos are not massless.

7. Jun 20, 2015

### WannabeNewton

Actually they are in the standard model so Orodruin is correct. Massive neutrinos are part of BSM.

8. Jun 20, 2015

### Orodruin

Staff Emeritus
Neutrinos are massless in the standard model as it does not contain a right-handed neutrino and a neutrino mass cannot be added with any renormalisable and gauge invariant operator. That neutrinos are massive is one of the few indications we have that the standard model does not give the whole picture. This is not very strange as the only gauge invariant dimension five operator you can add gives rise to neutrino masses after electroweak symmetry breaking.

9. Jun 20, 2015

Staff Emeritus
First, two answer the question asked, yes, the SM permits a massless particle that is not its own antiparticle: that is, an extension with these fields could be introduced without breaking something else. However, there is no such particle. My statement is a bit like saying zoology permits a black giraffe - but there aren't any. No reason you couldn't have one. We just don't.

Second, the statement that neutrinos are massless in the SM is something that one heard a lot more after the discovery that neutrinos have mass than before it. Before it, most people probably thought that the SM meant that neutrinos were Dirac particles like every other fermion, and their right handed components were sterile, just as you would expect. Certainly that's what I thought, and what my professors thought. I don't think anyone would have considered this model non-SM at the time.

10. Jun 20, 2015

### Orodruin

Staff Emeritus
There is absolutely no reason to expect this. The introduction of the right-handed neutrino field allows it a Majorana mass term, which immediately leads to neutrinos being Majorana, or at the very least pseudo Dirac. This was well known long before the discovery of neutrino oscillations (the seesaw mechanism was introduced in the 70s).

The fact is that the standard model, as it stands, does not include a right-handed neutrino and therefore no neutrino masses. That this is not a correct description of nature is a different story. Also note that the standard model was only brought into this after the OPs second post, where the standard model is first mentioned. There are two separate questions here: 1) Does the standard model contain a massless particle which is not its own anti-particle? and 2) do we know of any such particle?

The answer to the first question is yes and as I mentioned earlier, depending on how you consider particles with different colour charge, you might not even need the neutrino to answer this. The second question excludes the neutrino. The problem as i see it is that the OP introduced the question as "so the standard model allows". The standard model does not say anything about hypothetical particles or fields which are not part of it.

11. Jun 21, 2015

Staff Emeritus
I was there. That's what people were thinking back then. Another thing that will probably shock you is that they idea that one should work with Weyl fields instead of Dirac fields hadn't caught on, and the chiral nature of the weak force was placed in the coupling, not in the fields themselves. That's just how it was.

While I will agree that the SM doesn't include undiscovered fields, it certainly permits some, in the sense that some are excluded (a massive vector that doesn't couple to the Higgs) and some are not excluded.

12. Jun 21, 2015

### Orodruin

Staff Emeritus
But this is exactly what I would argue against. The SM as it stands is what it is, with a well defined field content. As such it does not permit any new fields as introducing new fields changes the model and then it is no longer the SM but a SM extension. I guess this is mostly semantics though.

13. Jun 21, 2015

### referframe

Interesting discussion. Learning Particle Physics and the Standard Model is on my bucket list.

Question: Is the original Dirac Equation still relevant to my question? Because, during the derivation of that equation, the matter-antimatter duality arises only after the mass term is introduced - when the 2X2 Pauli Matrices are not sufficient and the 4X4 Dirac Matrices become necessary. This seems to imply that anti-matter is strictly mass related.

14. Jun 22, 2015

### samalkhaiat

There is no particle-antiparticle “duality”, because they both interact with equal strengths. The two (particle-antiparticle) are related by charge conjugation and being massive or massless has no relevance here. As for your original question, it is now well established that all SM particles are in fact massless and some of them appear massive to us because of the way they move through the Higgs field.

15. Jun 24, 2015

### BiGyElLoWhAt

Test

16. Jun 24, 2015

### BiGyElLoWhAt

I don't know why, but I can't post my arxiv link :-(
The paper is called "six observations consistent with the electron neutrino being a tachyon with mass $m_{e_{\nu}}^2 = -0.11 \pm .016 \text {eV}$

Last edited: Jun 24, 2015
17. Jun 24, 2015

### BiGyElLoWhAt

Anyway, that's a paper I ran into during my particle physics class last fall. It had no application to the class, and my prof basically told me to shut up when I asked about it, but I thought it was pretty neat.

18. Jun 24, 2015

### BiGyElLoWhAt

One more shot at the hodge podge link :-/
htt p://arxi v.org/abs/1408.2804
Sorry about the spaces, the link exists, google neutrinos tachyons arxiv and its the first link. I'm not sure why I can't link to it.