"Existence of Axion" -- R.N. Mohapatra
Rabindra N. Mohapatra (photo courtsey: U. of Maryland)
[This is an invited article. In a recent publication in Physical Review Letters, R.N. Mohapatra (U. of Maryland) and Salah Nasri (U. of Florida) have put forward a theory that can reconcile conflicting results from two experiments that tried to test the existence of "axion" -- an ultralight particle that could make up dark matter. We thank Prof. Mohapatra for contributing this article on our request. -- 2Physics.com Team]
Axions were proposed by Roberto Peccei and Helen Quinn as a way to solve one of the fundamental mysteries of nuclear forces i.e. an inordinately large amount of CP violation in the otherwise successful theory of these forces, Quantum Chromodynamics (QCD) proposed by D. Gross, F. Wilczek and H. D. Politzer. Since their debut into the world of theoretical physics, axions have also been found useful in another context: being ultralight particles (believed to be a billion times or more lighter than the electron) they are capable of populating the Universe so abundantly that they could be candidates for the dark matter of the Universe and thereby resolve another fundamental mystery of cosmology.
Salah Nasri (photo courtsey: U. of Florida)
Because of these twin attributes (solving the problem of QCD and being a candidate for dark matter), considerable amount of research is being devoted to establishing the existence of axions. One of their key properties is that they couple to two photons (one being the magnetic and the other the electric component of light). Therefore interaction of laser beams with strong magnetic fields is considered to be an efficient way to search for them.
Two recent attempts that use this technique to search for axions are the CERN CAST (CERN Axion Solar Telescope) experiment and PVLAS experiment at INFN-LNL. The CAST experiment searched for axions produced by light-by-light collision at the center of the Sun and gave a negative result setting strong limits on the axion-photon coupling and its mass. The PVLAS experiment on the other hand looked for axions produced by laser-magnetic field interaction in the laboratory and seems to have a positive evidence for an axion like particle. Their observations can be understood only if the axion-photon coupling are considerably larger than the upper limits set by the CAST result. This has posed a major challenge for theory and in the very least implies that the axion solution to the problems of QCD may be much more complex than previously envisioned or the PVLAS experiment could be the result of completely new kind of phenomenon, not related to the axion.
In a recent Phys. Rev. Lett. Paper , we have proposed a new way to reconcile the CAST and the PVLAS results. We use the axion possibility in our approach except the theory has several new features compared to the conventional axion models. We use the phenomenon of phase transition so well known in the study of condensed matter physics (e.g. loss of magnetism of ferromagnets at high temperature). Our basic observation is that the axion photon coupling is not a primordial coupling but is induced by the formation of a vacuum condensate. Therefore the strength of the coupling depends on the environment temperature.
Note that the solar axions are produced at a very high temperature of about 10 million degrees in the core of the Sun whereas the PVLAS axions are produced at room temperature. Therefore if the vacuum condensate responsible for axion-photon coupling undergoes phase transition to zero value in the solar core due to its high temperature, there would be no axion production in the solar core explaining the CAST result. On the other hand, the PVLAS experiment is taking place at the room temperature and therefore the vacuum condensate has nonzero value and the axion-photon coupling is present giving rise to the PVLAS signal for the axion.
This idea is consistent with all known experimental observations in particle physics and astrophysics. We predict that the axion must be accompanied by a twin particle with mass about 100 times that of the electron which undergoes the condensation and is responsible for our effect. It can be produced in the decay of heavy quark bound states, which can provide a way to test our model.
 P.Sikivie, Phys. Rev. Lett. 51, 1415 (1983)
 S.Andriamonje [CAST Collaboration], arXiv:hep-ex/0702006.
 E.Zavattini et al. [PVLAS Collaboration], Nucl. Phys. Proc. Suppl. 164, 264 (2007).
 R. N. Mohapatra and S. Nasri, "Reconciling the CAST and PVLAS Results", Phys. Rev. Letters 98, 050402 (2007) Link to Abstract