# Do axions have an anti-particle?

Axions are the particles that let energy flash between a matter particle and its anti-particle. But according to quantum field theory, every particle has an anti-particle (besides the majorana fermion). So if axions have an anti-particle, why don't they react?

By the way, I am a very smart middle schooler.

• I suggest that you be more considerate towards physicists who spend time explaining something to you - I refer to anna v's explanation which you dismissed, while clearly not understanding what she wrote. – Helen - down with PCorrectness Dec 2 '19 at 10:31

## 2 Answers

The axion is a hypothetical particle; no axion has been observed. (Neither have any particle-antiparticle oscillations.) Since we don't know the quantum numbers of any axion, we don't know whether they are distinct from their antiparticles or not.

I'm pretty sure that most models of the axion don't have any quantum numbers which would change sign under the charge-conjugation operation, which converts particles to antiparticles. Such an axion would, like the photon and the $$Z^0$$, be its own antiparticle.

• Dude axions exist breh – Peter Barendt Nov 25 '19 at 18:29
• Citation needed. If you're thinking about the recent news stories about a possible discovery experiment, you might read my discussion of those results in response to this other question. The status of the axion is not as simple as "dude breh." – rob Nov 25 '19 at 18:33
• @mithusengupta123 I don't know what a "PQ charge" is. Axions would have zero electric charge, but one model for the axion is that it's the lowest-mass supersymmetric particle, and the many supersymmetric particles are distinguished by their own sets of quantum numbers. That's probably a better topic for a follow-up question than for a comment discussion. – rob Nov 26 '19 at 5:34
• @rob Doesn't axions of QCD hypothesized to solve the strong CP problem come from $U(1)$ Peccei-Quinn (PQ) symmetry breaking? – mithusengupta123 Nov 26 '19 at 5:37
• @mithusengupta123 I don't know that I've heard about Peccei-Quinn symmetry breaking. (I'm more of a nuclear person than an axion person, and I've had low research engagement for a few years.) If there's a quantum number associated with that symmetry which would make a probable model for an axion be not its own antiparticle, consider writing a more correct answer to this question, or proposing an edit to mine. – rob Nov 26 '19 at 5:52

Rob's answer is giving the correct physics, but since you are a "smart middle schooler" I would like to address the rest of your question. To start with it will be good to increase your background on axions by reading the wikipedia article. , for encyclopedic information.

Axions are the particles that let energy flash between a matter particle and its anti-particle.

In particle physics very strict mathematical rules describe the interactions between particles. Particles are on the same status as antiparticles and nothing particular "flashes" unless there is an interaction. Interaction means energy and momentum is exchanged. Usually particles and antiparticles annihilate when they interact, and a plethora of Feynman diagrams describes pictorially the mathematics of their interaction.

But according to quantum field theory, every particle has an anti-particle (besides the majorana fermion).

Particles can be antiparticles of themselves, as for example the photon. In general bosons are antiparticles of themselves if they are neutral, and the axion is neutral.

So if axions have an anti-particle, why don't they react?

Interactions happen with energy and momentum exchanges between particles, and theory up to now can predict the probability of various interaction products coming out using field theory and the Feynman diagrams. The existence of antiparticles does not have anything to do with interactions, which need energy and momentum to happen.

In some theories, if the axion has mass it can decay into photons , the energy supplied by its mass.

This is a Feynman diagram for an axion interaction with a magnetic field, that can allow calculating the probability of this happening within a specific theoretical model:

Here is the decay, where the mass of the axion provides the energy.

One can then run an experiment trying to detect photons that could be attributed to axion interactions and decays.

Maybe your flashing statement refers to vacuum loop feynman diagrams . Those diagrams cannot exist by themselves, they need an input energy and momentum, they are higher order (much smaller probability) Feynman diagrams that have to be added to calculations to make them more accurate.

A lot of study in mathematics and physics is needed to be able to understand the present model of particle physics interactions.

• Wikipedia is junk and since axions are the particles that let the energy flash happen between particles and their anti-particle nobody really knows if they do, kind of like that neutrino thing – Peter Barendt Nov 26 '19 at 14:13
• It makes no sense yet it does at the same time though – Peter Barendt Nov 26 '19 at 14:14
• Wikipedia is not junk, it is for most physics an easy introduction for people who are not into the mathematics of physics. "Energy flashes" you imagine are science fiction. There are no energy flashes defined in mainstream physics vocabulary for particle physics. – anna v Nov 26 '19 at 15:30
• just a hint : for smar middle t schoolers to become smart physicist a lot of elbow grease is needed, knowledge is not automatic. – anna v Nov 26 '19 at 15:41
• I know and the flash I was referring to was the energy flash created when a particle meets its anti-particle – Peter Barendt Dec 3 '19 at 14:54