Less than three weeks after physicists spotted a tentative signature for axions, another research group is claiming to have “the strongest evidence yet” of the hypothetical particles. The new evidence, which again appears in astrophysical data, puts more pressure on Earth-bound experiments to find a signal.

Axions were first proposed in the late 1970s to solve an issue in particle physics known as the strong-CP problem. If they exist they would be very light and interact very weakly with matter, but these properties have made them difficult to find. No experiment on Earth has yet discovered any evidence of the particles.

However, researchers have predicted that axions could help electromagnetic radiation travel from distant sources — such as the bright “active” nuclei of certain galaxies — and that this process could reveal their existence. Indeed, astrophysicists know that the high-energy photons generated by active galactic nuclei (AGN) should be unable to travel intergalactic distances because they are absorbed by the universe’s opaque background of microwave radiation — yet telescopes still detect them.

Earlier this month, a group led by Malcolm Fairbairn of King’s College, London, found statistical evidence hinting that axions are the reason why we can detect the high-energy photons. The evidence suggested that the photons are temporarily converting to axions, which can bypass the microwave background without absorption.

Testing the scatter

Now, Anne-Christine Davis of Cambridge University, Clare Burrage of the DESY lab in Hamburg and Douglas Shaw of Queen Mary University of London have different astrophysical evidence for such an axion–photon conversion mechanism. They have looked closely at the “luminosity relations” for compact astrophysical sources that describe how X-ray or gamma-ray luminosities are related to certain properties, such as the peak energy output or the luminosity at lower frequencies.

The luminosity relations are not exact for all sources. There is normally a deviation or “scatter” about the predicted relation, which takes into account measurement error and other effects. Davis and colleagues have compared the scatter data for compact astrophysical sources with both a traditional bell-shaped “Gaussian” distribution and a distribution that accounts for axion–photon conversion. Although data for two types of source — gamma-ray bursts and blazars — roughly fitted the Gaussian, data for 77 AGN were a much closer match of the axion distribution.

The researchers also created what they call “fingerprints”, which exaggerate features of the scatter data of all the AGN for comparison with both the Gaussian and axion models. Visually, it was the axion model that best resembled the real data (arXiv:0902.2320).

Trusting the Gaussian

Davis’s group is careful to state that there could be another, unknown reason why the Gaussian distribution is a poor fit for AGN, a point echoed by other researchers specializing in axion physics. “There is no reason for the astrophysics of AGN to be well described by a Gaussian scatter in [luminosity relations],” explains Aaron Chou, a spokesperson of the GammeV axion experiment at Fermilab. “So, while the results of the axion-like particle model are suggestive, they are by no means conclusive. The model is however very interesting, and I hope the authors, or others, will be able to find ways to test it.”

Like the axion possibly revealed by Fairbairn and others earlier in the month, the type of particle producing Davis and colleague’s results would couple with light too weakly to be a solution to the strong-CP problem. Moreover, it would be too light for dark matter.

However, the particle could be a solution for dark energy, the entity that physicists believe makes up more than 70% of the universe’s mass-energy content and causes the universe to expand at an accelerated rate. This is because, rather than being an axion, the particle could be something similar: a chameleon. Like their reptilian namesakes, chameleon particles adjust their properties to suit the local environment, in the sense that their interactions with matter are stronger and farther reaching in a vacuum than in a dense material. In this way chameleon particles could be responsible for vast regions of the empty cosmos being pushed apart.

Davis told physicsworld.com that, assuming that there is a particle producing the effect, it might be possible to distinguish between axions and chameleons by looking at the polarization of the X-rays coming from the AGN. Both types of particle should rotate the polarization, but the difference in coupling to photons means the polarizations should differ by 90°. Scientists have not actively recorded X-ray polarizations since the 1970s, though there are several missions to make such measurements on the drawing board.

New physics?

Still, it should be feasible to look for the particles on Earth. Konstantin Zioutas, spokesperson for the CAST axion experiment at CERN and a researcher at the University of Patras in Greece, says that the group’s work motivates his team to upgrade CAST. “This is certainly a clever idea the authors have followed theoretically and substantiated to some degree also observationally,” he says, adding: “However, an independent verification is in order to exclude the possibility that conventional physics is mimicking new physics.”