In search of the axion, a hypothetical elementary particle
For some time now, physicists have been thinking about an elementary particle that has very little mass, no electric charge and no spin (quantum angular momentum). It would interact very little with other particles because of these properties and would therefore be a good candidate for dark matter, which is characterized by just that. But the axion is also used in physics because in the neutron, a neutral nuclear particle, the charge of the quarks of which it is composed is so perfectly distributed that it is not at all apparent to the outside world that there are balancing charges inside the neutron.
Physically expressed: The neutron should have an electric dipole moment, which is not measurable. This can have two reasons: The quantum chromodynamics is wrong, from which the dipole moment results. Against it speaks that it works otherwise quite well. Or there is an additional elementary particle, the axion. Against it speaks that one could not detect it up to now.
So far. A new study led by a theoretical physicist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) now suggests that axions may be the source of unexplained, high-energy X-ray emissions surrounding a group of neutron stars. First theorized in the 1970s, physicists expect axions to be produced in the core of stars and to convert into particles of light (photons) in the presence of a magnetic field.
A collection of neutron stars known as the Magnificent 7 provides an excellent testing ground for the possible presence of axions because these stars have strong magnetic fields, are relatively close – within hundreds of light-years – and should produce only low-energy X-rays and ultraviolet light. In this case, however, the researchers detected high-energy X-ray emissions, the source of which is unclear.
«They are known for being very ‘boring,’ and in this case that’s a good thing,» says Benjamin Safdi of the Berkeley Lab Physics Division theory group, who led a study published Jan. 12 in the journal Physical Review Letters that details the axion explanation for the excess.
If neutron stars are a type known as pulsars, they would have an active surface that emits radiation at different wavelengths. That radiation would show up across the electromagnetic spectrum, Safdi said, and could drown out the X-ray signature the researchers found, or it would produce high-frequency signals. But the Magnificent 7 are not pulsars, and no such radio signal has been detected. Other common astrophysical explanations also don’t seem to hold up to the observations, Safdi said.
Safdi and his collaborators say it is still possible that a new explanation for the observed X-ray excess will emerge. However, they remain hopeful that such an explanation will lie outside the Standard Model of particle physics and that new ground- and space-based experiments will confirm the origin of the high-energy X-ray signal.
«We are pretty confident that this excess exists, and very confident that it is something new,» Safdi says. «If we were 100 percent sure that what we were seeing was a new particle, that would be great. That would be revolutionary in physics.» Even if the discovery turns out not to be related to a new particle or dark matter, he says, «It would tell us so much more about our universe, and there would be a lot to learn.»
Raymond Co, a postdoctoral researcher at the University of Minnesota who worked on the study, says, «We’re not saying we’ve already made the axion discovery, but we are saying that the extra X-ray photons can be explained by axions. It’s an exciting discovery of the excess in the X-ray photons, and it’s an exciting possibility that’s already consistent with our interpretation of axions.»
The next step would then be to study white dwarf stars. They would be an excellent place to look for axions because they also have very strong magnetic fields but are X-ray free. Safdi is also enthusiastic about ground-based experiments such as CAST at CERN, which works as a solar telescope to detect axions converted to X-rays by a strong magnet, and ALPS II in Germany, which uses a strong magnetic field to cause axions to convert to light particles on one side of a barrier when laser light hits the other side of the barrier.
In general, axions are getting some attention at the moment, as a number of experiments have failed to find evidence of WIMPs (weakly interacting massive particles), another promising dark matter candidate. However, the picture that physics currently has of axions is not clear. In fact, it could be an entire family album: There could be hundreds of axion-like particles (or ALPs) that make up dark matter, and string theory – a candidate theory for describing the forces of the universe – holds open the possible existence of many types of ALPs.