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PARTICLE PHYSICS: ON DETECTING THE FIRST AXION

The following points are made by Steve Lamoreaux (Nature 2006 441:31):

1) New work [1] reports that a magnetic field can be used to rotate the polarization of a light wave in a vacuum. Although this is the first experimental evidence for such an effect, there is a well-rehearsed, but controversial, explanation for it: the existence of a never-before-seen, chargeless, spinless and near-massless particle -- the axion. Has the elusive axion finally allowed itself to be glimpsed?

2) The idea that static electric and magnetic fields alter the optical properties of matter, whether solid, liquid or gas, is not new. In 1845, Michael Faraday (1791-1867) showed that the direction of polarization of light changes in a medium permeated by a magnetic field. Similar demonstrations followed: of the Cotton-Mouton and Kerr effects, for instance, in which the propagation speed of a light wave can be made to vary according to the orientation of its polarization with respect to an applied magnetic or electric field, respectively.

3) That static electromagnetic fields can be used to alter the properties of empty space might seem surprising. The culprit is quantum electrodynamics (QED), the quantum field theory that expresses the electromagnetic interaction in terms of the exchange of quanta of energy -- photons -- between particles. It was recognized in the early 1930s that the rules of QED allowed the creation and annihilation in an electro-magnetic field of short-lived "virtual" charged-particle pairs consisting of an electron and its antiparticle, the positron. The resulting "self-interaction" of the field introduces a nonlinearity in the governing equations of electro-magnetism (Maxwell's equations) analogous to that induced by the presence of matter. The vacuum self-interaction is further complicated by the fact that, given the presence of sufficiently energetic photons, real (as opposed to virtual) electron-positron pairs can be created. Nevertheless, by 1936, vacuum analogues of various optical effects in matter had already been calculated[2,3].

4) So what other particles might the vacuum be hiding? In the late 1970s, a suitable candidate was postulated to solve the so-called strong CP problem. This problem pops up as a term in the field equations of quantum chromodynamics (QCD) -- the equivalent of QED for the strong nuclear force, which binds together the innards of neutrons and protons. This term is not symmetric when, in a process involving the strong force, particles are replaced by antiparticles (charge conjugation; C) and the process is viewed in a mirror (parity inversion; P). The degree of this "symmetry breaking" can be parametrized by an angle that, according to a limit obtained from measurements of the electric dipole moment of the neutron, is less than a billionth of a radian. Strong-force interactions covered by QCD thus hardly seem to break CP symmetry at all. This is an odd fact, because interactions involving the weak nuclear force (which can be incorporated with the electromagnetic force in QED) observably break CP symmetry. As there is no a priori reason for the discrepancy between these forces, Roberto Peccei and Helen Quinn proposed [4] a mechanism implying the existence of another electrically neutral particle that would force the QCD symmetry-breaking angle -- which could otherwise be as large as that in QED -- to become zero. This particle cleaned up the strong CP problem, so Frank Wilczek dubbed it the "axion", after a laundry detergent that enjoyed limited commercial success in the United States[5].

References (abridged):

1. Zavattini, E. et al. Phys. Rev. Lett. 96, 110406 (2006)

2. Heisenberg, W. & Euler, H. Z. Phys. 98, 714-732 (1936)

3. Bialynicka-Birula, Z. & Bialynicki-Birula, I. Phys. Rev. D 10, 2341-2345 (1970)

4. Peccei, R. & Quinn, H. Phys. Rev. D 16, 1791-1797 (1977)

5. Wilczek, F. http://nobelprize.org/physics/laureates/2004/wilczek-lecture.pdf (2004)

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