Shining light thru the sun: evidence of axions?

[marcus]

this just appeared in the peer-review journal Physical Review Letters. I noticed the preprint of it on arxiv last year and thought it was pretty far out. Here is the revised October 2006 preprint:

http://arxiv.org/abs/astro-ph/0610844
Shining light through the Sun
Malcolm Fairbairn, Timur Rashba, Sergey Troitsky
Published in PRL under title "Transparency of the Sun to gamma rays due to axionlike particles[/color]"
(Submitted on 30 Oct 2006 (v1), last revised 19 May 2007 (this version, v2))

"It is shown that the Sun can become partially transparent to high energy photons in the presence of a pseudo-scalar. In particular, if the axion interpretation of the PVLAS result were true then up to 2% of GeV energy gamma rays might pass through the Sun, while an even stronger effect is expected for some axion parameters. We discuss the possibilities of observing this effect. Present data are limited to the observation of the solar occultation of 3C 279 by EGRET in 1991; 98% C.L. detection of a non-zero flux of gamma rays passing through the Sun is not yet conclusive. Since the same occultation happens every October, future experiments, e.g. GLAST, are expected to have better sensitivity."

The relevance for us in Cosmo forum is that AXIONS if they can be proven to exist MIGHT BE DARK MATTER
and this quasar eclipse experiment is a TACTIC FOR SEEING IF AXIONS ARE REAL. People are getting clever about ways to get evidence of the existence of dark matter, if it exists. I think it's exciting.

The PhysWeb news site picked up on the story and had this to say:

"Quasar eclipses could clarify axion mystery
Physicists have devised an experiment that could clarify whether hypothetical ultralight particles called axions -- which some claimed were glimpsed in the lab last year -- exist for real. It involves using space telescopes to check whether gamma rays from a quasar billions of light years away can travel through the Sun by interacting with its magnetic field (Phys. Rev. Lett. 98 201801)."[/color]

PhysWeb requires a subscription, but here is an alternative source
http://www.scicentral.com/P-02phys.html
called SciCentral.

 

[Garth]

Axions ruled out by PVLAS There go the axions...

2 July 2007

The existence of a hypothetical particle called the axion has been put into further doubt now that the team that first claimed its discovery has failed to reproduce their results. Physicists working on the PVLAS experiment in Italy say that the tiny rotation in the polarization of laser light that they reported last year does not support the existence of axions, but rather is an artefact related to how the experiment had been performed (arXiv:0706.3419v1).
Axions are ultralight particles that were first postulated in the 1970s to resolve a discrepancy between experimental findings and the theory of the strong force. In March 2006, the PVLAS team shone a laser beam through a strong magnetic field of 5.5 T in a vacuum and saw that the beam's polarization rotated slightly. At the time many physicists thought that this was due to an ultralight particle coupling with photons in the beam, and so heralded it as the first glimpse of the axion.

Now, the PVLAS team has repeated the original experiment at two different magnetic field strengths. While the rotation was again observed at the original field of 5.5 T, no effect was seen at 2.3 T – leading the team to conclude that the rotation is an instrumental effect related to the magnetic field strength.

The latest news from Italy should come as a relief to physicists who believe that axions could make up dark matter. This is because the PVLAS axion appeared to couple too strongly to light to be a suitable candidate for dark matter.
The null result also puts PVLAS line with an experiment at CERN called CAST, which has been trying to convert solar photons into axions in a 10-m long test magnet. CAST has found no evidence for axions at the coupling strength implied by the 2006 PVLAS result.

Physicists are now gearing up to search for axions by studying gamma rays from a distant quasar that will soon be passing through the intense magnetic field of the sun. Such observations can be made every October when the Sun comes between the Earth and the quasar 3C 279. During this alignment, a small number of the quasar's gamma rays could be converted into axions at the far surface of the sun. The axions should then travel through the Sun unhindered, only to be converted back into gamma rays when they emerge on the near side. Unconverted gamma rays, however, would be blocked by the sun and therefore any gamma rays from the quasar that are detected on Earth would be seen as evidence of axions.

Garth

 

[marcus]

Personally I've never been a fan of axions, but Frank Wilczek speaks well of them

As I read the PVLAS announcement they did not actually RULE OUT the existence of axions.

They simply RETRACTED some supporting evidence. A few months back they interpreted some data as positive indication, and they now rescind that interpretation.

For better or worse, axions have not been ruled out, they just have not been seen yet. And people go on looking for them.

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just as a side-comment, when it comes to big questions like about inflation dark energy dark matter conditions prior to big bang...
when it comes to big questions, I expect more progress in Quantum Gravity than I do in Particle Physics.

Bojowald just conjectured a rather interesting explanation of acceleration just using the ingedients of LQG. His proposed explanation would make a "dark energy" field unnecessary. And he has ideas of how to test it.

I think that by looking at the fundamental degrees of freedom that underly space time and matter, people stand a better chance of coming up with new insights than they do by going over and over the same particle theory ground (with its basis in an old idea of spacetime).

You get to a point where finding a new particle doesn't help, and going out looking for a new particle is not the best way to address the main questions.

But all the same good luck to them in their hunt for axions.