Tuesday, July 17, 2007


There's been a bit of news recently about axions.

But first, what is an axion, and why should anyone care? Don't we have enough oddball particles already? Perhaps we do, but regardless of that, the properties axions are hypothesized to possess would be a good fit for what is needed in a particle that could make up at least some dark matter.

Specifically, the theory behind axions predicts that they would be rather light, but not massless – between 10-6 and 10-2 eV/c2. For comparison, the mass of an electron is about 510,999 eV/c2. On the other hand, neutrinos (all three types) are known to have mass, and though this mass isn't well determined, axions would be in the same range.

Just as axions interact very weakly with gravity, they should interact very weakly, if at all, with the weak and strong nuclear forces. In particular, axions should have no electric charge. These properties make them good dark matter candidates, unlike neutrinos, which do feel the weak force.

On the surface, the most recent news, concerning the failure of an experimental attempt to detect axions, appears to be discouraging. But the silver lining is that if the experiment had actually detected axions, it would imply that they couple too strongly to photons to be a dark matter candidate. For that reason, this failure has been greeted with a bit of relief.

You see, one other expected property of axions is that in the presence of a strong magnetic field, axions could transform into photons, or vice versa. This makes it possible to design certain experiments for detecting axions, as we shall see.

Theoretically, the reason axions were predicted to exist in the first place has to do with breaking of charge-parity (CP) symmetry by the strong nuclear force. Or rather, the apparent failure of CP symmetry breaking by the strong force.

CP symmetry breaking was first discovered in 1964 in connection with the weak force. Even there, the effect is small, but by now it has been verified and measured repeatedly. Quantum chromodynamics (QCD) is the quantum field theory of the strong force, and it is similar enough to the theory of the weak force that there is no apparent reason CP symmetry breaking isn't observed with the strong force.

There is a parameter in the equations called θ which describes the amount of symmetry breaking. To accord with experimental results, θ must be fine-tuned to be extremely close to 0, even though it could have any value from 0 to 2π. Physicists do not like such fine tuning, and so they have given the name "strong CP problem" to the lack of CP breaking by the strong force, or alternatively the smallness of θ.

The solution to this problem that has been most popular with physicists is based on what is called the Peccei-Quinn mechanism, after Roberto Peccei and Helen Quinn, who were both at Stanford in 1977 when they came up with theory. (Trivia note: I sat in on a quantum field theory course given by Peccei in 1974. As I was in mathematics, it didn't make a whole lot of sense to me then, what with all the Feynman integrals and such flying around, oblivious to their lack of rigorous mathematical definition. I also recall the class meeting in November 1974 when the discovery of the J/Psi particle was announced. I didn't understand what all the exceitement was about, either. Future astronaut Sally Ride, then a physics grad student, was also in the class.)

Anyhow, the Peccei-Quinn proposal was to make this parameter θ into a quantum field (meaning it could have different values at different points). Along with this field, there should be a new global symmetry (Peccei-Quinn symmetry) that, however, is spontaneously broken. As Frank Wilczek and Steven Weinberg then showed, this implies the existence of a particle – which Wilczek called the axion, because it "cleaned up" the theory. (Axion was the name of a popular brand of laundry soap.) They also showed this would be a satisfactory solution to the strong CP problem.

It would be even more satisfactory if experiments could actually detect axions. But in the 30 years since then, this has not happened.

That basically brings us to the latest news about axions. Because axions and photons can turn into each other, it turns out that if a beam of polarized light is passed through a strong magnetic field a small rotation in the direction of polarization should occur, due to interaction of the photons with the magnetic field, creating real or virtual axions.

The PVLAS laboratory in Italy has been conducting such an experiment for several years. For awhile the reasearchers thought they has obtained positive results. Sadly, no. Just a couple weeks ago the researches reported their earlier results couldn't be reproduced:

Axions ruled out by PVLAS

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.

But the silver lining is:
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.


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