# Axion signature in the presence of strong magnetic fields?

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1. Oct 16, 2014

### sikrut

So there was this article published today that some researchers may have found a dark matter signature emanating from the sun in the form of xr-ray emission from axion interactions with earths magnetic field.

Article: http://www.theguardian.com/science/2014/oct/16/dark-matter-detected-sun-axions

Paper: arxiv.org/abs/1403.2436

Wikipedia quote: "Axions are predicted to change to and from photons in the presence of strong magnetic fields, and this property is used for creating experiments to detect axions."

How can a particle interact with a magnetic field without having an electric charge? I assume it must be some strong/weak interaction, but how does that work?

2. Oct 16, 2014

### Haelfix

Think of the neutral pion, and its interaction with the electromagnetic field.

3. Oct 16, 2014

### sikrut

You mind if I ask you what is a pion and how it interacts?
(I'm actually doing QM homework right now, or else i'd read up myself :P )

4. Oct 17, 2014

### Staff: Mentor

A neutral pion still consists of charged quarks, while an axion does not.

Axion are introduced as particles with a coupling to photons (and therefore an interaction with electromagnetic fields). There is no deeper "why" reason - this is a model and you can look if you see those hypothetical particles in nature. If you want to count that as electromagnetic interaction is a matter of semantics.

5. Oct 18, 2014

### Haelfix

Yes, although there are some subtleties here and there is a sense as to why there is a coupling.

This goes well beyond the scope of the original poster/question, but at the level of effective field theory, the coupling between the strong CP axion and the electromagnetic field (well the two photon decay vertex) is completely identical in form as that arising from the neutral pion. The actual electromagnetic coupling of the axion itself is determined by the electromagnetic anomaly of the PQ current, and in fact, b/c the quantum numbers are identical there is a mixing of the bare axion part (which takes contributions from the PQ symmetry breaking and is a function of the PQ charges of the right handed quark fields) with the other light pseudoscalars mesons like the neutral pion and the Eta.

The analysis of the EM effects in a strong inhomogenous magnetic field as a consequence of the similarities will of course involves similar equations.

6. Jan 28, 2015

### ChrisVer

Well, to make this as correct as possible, I would rephrase the "axion are introduced as... with coupling to photons" . The axions couple to photons via anomalies and because they can interact with quarks. It's the same thing as the pion-0 (you don't have to think that the pion consists of quarks, but rather take it as an axial current / pseudo-scalar).

Similarily the axions couples to quarks (into a triangle diagram) via EM-anomalies and produces the 2 photons. Apart from that, there is also a coupling of axions to pions and etas (or etas-primed) which also contributes to the axion-2 photon coupling. The triangle diagram is then reduced to a vertex interaction by replacing all the mechanisms that occur in the triangle with the axion-2 photon coupling $g _{a \gamma \gamma}$.

As for answering the OP question, that is impossible to understand by someone who doesn't know basic stuff in QFT. Everything I can tell you, is that that's how it works. But in general it's good to think that first the axion couples to light quarks (which carry PQ charge) and then these quarks generate the photons.