Axion

From Wikipedia, the free encyclopedia

You have new messages (last change).
Axion
Status: Theoretical
Theorized: 1977, Peccei and Quinn
Mass: 10−6 to 10−2 eV/c2 (theoretically)
Electric charge: 0 (theoretically)
For other uses, see Axion (disambiguation).

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 2006, an experimental search by the PVLAS collaboration reported results suggesting axion detection. Not only has that not yet been confirmed by other searches, but it might in fact contradict the results of other experiments (e.g. CAST) which have been confirmed.

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".

Contents

[edit] 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 an 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 Nambu-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-Nambu-Goldstone boson.)

[edit] Properties

One theory of axions relevant to cosmology had predicted that they would have no electric charge, a very small mass in the range from 10−6 to 10−2 eV/c2, and very low interaction cross-sections for strong and weak forces. Because of their properties, axions interact only minimally with ordinary matter. 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.

In supersymmetric theories the axion has both a scalar and a fermionic superpartner. The fermionic superpartner of the axion is called the axino, the scalar superpartner is called the saxion. In some models, the saxion is the dilaton.

[edit] Experimental searches

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

The Italian PVLAS experiment passes polarized light through a magnet that produces a magnetic field of 5 tesla, searching for a small anomalous rotation of the direction of polarization. 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.[1]

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.[1] 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 way to search for axions is by conducting so called "light shining through walls" experiments.[2] By passing a beam of light through an intense magnetic field, a few photons should be converted into axions. A second intense magnetic field on the far side of a wall, through which the axions should pass readily while the photons are blocked, should convert some of the beam back into photons. Both of these processes are low efficiency, necessitating a very high initial photon flux.

[edit] Cosmological implications

Theory suggests that axions were created abundantly during the Big Bang. Because of a unique coupling to the instanton field of the primordial universe (the "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation. This robs all such primordial axions of their kinetic energy.

If axions have low mass, thus preventing other decay modes, axion theories predict that the universe would be filled with a very cold Bose-Einstein condensate of primordial axions. Hence, depending on their mass, axions could plausibly explain the dark matter problem of physical cosmology. Observational studies to detect dark matter axions are underway, but they are not yet sufficiently sensitive to probe the mass regions where axions would be expected to be found if they are the solution to the dark matter problem. The microwave cavity experiment known as ADMX recently ruled out an axion as light as about 10−6 eV. High mass axions of the kind searched for by Jain and Singh (2007)[3] would not persist in the modern universe and could not contribute to dark matter.

[edit] References

  1. ^ L. D. Duffy et al., A High Resolution Search for Dark-Matter Axions, Phys. Rev. D 74, 012006 (2006)preprint
  2. ^ A. Ringwald, Fundamental physics at an X-ray free electron laser, Invited talk at the "Workshop on Electromagnetic Probes of Fundamental Physics", Erice, Italy, October 2001 preprint
  3. ^ P. L. Jain, G. Singh, Search for new particles decaying into electron pairs of mass below 100 MeV/c2, J. Phys. G: Nucl. Part. Phys., 34, 129–138, (2007); doi:10.1088/0954-3899/34/1/009, (possible early evidence of 7±1 and 19±1 MeV axions of less than 10−13 s lifetime).
  • R. D. Peccei, H. R. Quinn, Physical Review Letters, 38(1977) p. 1440.
  • R. D. Peccei, H. R. Quinn, Physical Review, D16 (1977) p. 1791–1797.
  • S. Weinberg, Phys. Rev. Letters 40(1978), p. 223:
  • F. Wilczek, Phys. Rev. Letters 40(1978), p. 279
  • E. Zavattini, et al., Phys.Rev.Lett. 96 (2006) 110406, hep-ex/0507107

[edit] External links

Retrieved from "http://en.wikipedia.org../../../a/x/i/Axion.html"