English

Noun

axions

1. Plural of axion

The axion is a hypothetical elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the strong-CP problem in quantum chromodynamics (QCD). In 2005, an experimental search by the PVLAS collaboration reported results suggesting axion detection ; however new experiments performed by the PVLAS team exclude this result . The 2005 PVLAS results were problematic because compatibility with the negative results of other searches, such as CAST, as well as astrophysical limits, ruled out standard axion scenarios, while alternative hypotheses have been postulated by other researchers. .

The name was introduced by Frank Wilczek, co-writer of the first paper to predict the axion, after a brand of detergent—because the problem with QCD had been "cleaned up".

History

Reasons for prediction

As shown by Gerardus 't Hooft, the strong interactions of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits the violation of the combined symmetries of charge conjugation and parity, collectively known as CP. Together with effects generated by the weak interactions, the effective strong CP violating term, \bar\Theta, appears as a Standard Model input parameter—it is not predicted by the theory, but must be measured. However, large CP violating interactions originating from QCD would induce a large electric dipole moment for the neutron. (While the neutron is an electrically neutral particle, nothing prevents charge separation within the neutron itself.) Experimental constraints on the currently unobserved neutron's electric dipole moment imply that CP violation arising from QCD must be extremely tiny and thus \bar\Theta must itself be extremely small or absent. Since a priori \bar\Theta could have any value between 0 and 2π (the parameter is periodic), this presents a naturalness problem for the standard model. Why should this parameter find itself so close to 0? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the strong CP problem.

One simple solution exists: if at least one of the quarks of the standard model is massless, \bar\Theta becomes unobservable, i.e. it vanishes from the theory. However, empirical evidence strongly suggests that none of the quarks are massless and so the strong CP problem persists.

In 1977, Roberto Peccei and Helen Quinn postulated a more elegant solution to the strong CP problem, the Peccei-Quinn mechanism. The idea is to effectively promote \bar\Theta to a field (particle). This is accomplished by adding a new global symmetry (called a Peccei-Quinn symmetry) to the standard model that becomes spontaneously broken. Once this new global symmetry breaks, a new particle results and, as shown by Frank Wilczek and Steven Weinberg, this particle fills the role of \bar\Theta—naturally relaxing the CP violation parameter to zero. This hypothesized new particle is called the Axion. (On a more technical note, the axion is the would-be Goldstone boson that results from the spontaneously broken Peccei-Quinn symmetry. However, the non-trivial QCD vacuum effects (instantons) spoil the Peccei-Quinn symmetry explicitly and provide a small mass for the axion. Hence, the axion is actually a pseudo-Goldstone boson.)

Experimental searches

A number of experiments have attempted to detect axions, including at least one that has claimed positive results.

In the Italian PVLAS experiment polarized light propagates through the magnetic field of 5 T dipole magnet, searching for a small anomalous rotation of the direction of polarization. The concept of the experiment was first put forward in 1986 by Luciano Maiani, Roberto Petronzio and Emilio Zavattini , and If axions exist, photons could interact with the field to become virtual or real axions. This rotation is very, very small and difficult to detect, but this problem can be overcome by reflecting light back and forth through the magnetic field millions of times. The most recent PVLAS results do detect an anomalous rotation, which can be interpreted in terms of an axion of mass 1–1.5 meV. However, there are other possible sources for such an effect besides axions.http://cerncourier.com/main/article/46/8/10

Several experiments search for axions of astrophysical origin using the Primakoff effect. This effect causes conversions of axions to photons and vice versa in strong electromagnetic fields. Axions can be produced in the Sun's core when X-rays scatter off electrons and protons in the presence of strong electric fields and are converted to axions. The CAST experiment is currently underway to detect these axions by converting them back to gamma rays in a strong magnetic field.

The Axion Dark Matter Experiment (ADMX) at Lawrence Livermore National Laboratory searches for weakly interacting axions present in the dark matter halo of our galaxy. A strong magnetic field is used to attempt to convert an axion into a microwave photon. The process is enhanced using a tunable resonant cavity scanning the 460–810 MHz range, as determined by the predicted mass of the axion.

Another means of searching for axions is by conducting so called "light shining through walls" experiments, where a beam of light is passed through an intense magnetic field in an attempt to observe the conversion of photons into axions by allowing them to pass through an aluminium plate, blocking the passage of photons. However, these practices are of low efficacy, necessitate high initial proton flux, and those conducted by BFRS and PVLAS have been the subject of some further verification. A recent experiment had the necessary sensitivity to detect this effect if the PLVAS 2005-signal was due to axions; however, no effect was seen.

On 9 July, 2007, a paper submitted to arXiv by Carlo Rizzo, and thus required review. would not persist in the modern universe and could not contribute to dark matter.

References

Notes

Journal entries

External links

• January 28, 2007 news article by newscientist.com
• December 06, 2006 news article by physorg.com
• July 17, 2006 news article from Scientific American
• March 27, 2006 news article by PhysicsWeb.org
• November 24, 2004 news article by PhysicsWeb.org
• CAST Experiment
• CAST at MPI/MPE
• ADMX at University of Washington