The hypothetical axion is a particle that might help scientists work out where the bulk of dark matter may be held in the Universe. So far, there has been much talk about the search for another type of hypothetical particle, the weakly interacting massive particle (WIMP), and little attention has been paid to the lowly axion. WIMPs are very appealing to scientists as proving they exist will help patch some holes in quantum theory. What’s more, WIMP detectors need to be huge, large volumes of underground caverns filled with hi-tech sensors and cleaning fluid – this makes for a cool funding proposal; think up and grand idea, explain that it will prove our understanding of the Universe and then receive a multi-billion $/£/€ cheque (it’s not quite as easy as that, but there are socioeconomic and political reasons for building such an awesome structure).
So how do you go about finding an axion? Surely this exotic particle will need an even bigger detector, especially as it has zero charge, very low mass and cannot interact via the strong and weak nuclear forces? Actually, a large WIMP-type detector would be useless for axion detection. Fortunately axions have a neat interaction with magnetic fields that can be detected with existing instrumentation. What produces the strongest magnetic field in the Solar System? This is where the Sun can help out…
The Universe is made up of a huge proportion of dark matter. We know it is out there as we can indirectly deduce its existence through its gravitational effect on visible objects like stars and galaxies. But what exactly is “dark matter”? Dark matter is theorized to be made up from baryonic matter and non-baryonic matter. “Baryonic dark matter” may be held in large astronomical objects that emit little radiation, such as neutron stars or black holes (usually known as massive astrophysical compact halo objects, or MACHOs for short). “Non-baryonic dark matter” is stuff that isn’t made up of baryons (neutrons or protons). WIMPs and axions are considered to be two major non-baryonic particles, as are more well-understood particles such as neutrinos.
Non-baryonic matter is notoriously difficult to detect, as often these particles will be of low mass and zero charge. As axions are so small and fast moving, they cannot use the strong or weak force to interact with baryonic matter. WIMPs on the other hand can interact via the weak nuclear force as they are more massive and slower moving. This is why, although they are still very difficult to detect, by building a large chamber filled with fluid, occasionally WIMPS should (theoretically) hit ordinary molecules, sending out a spark of Cherenkov radiation. Axions are less likely to do this as their interaction cross section is much smaller (as they are low mass) and they are thought to travel very close to the speed of light.
Axions do however have an interesting quality; they are thought to turn into photons when passing through a strong magnetic field.
Hugh Hudson of UC Berkeley thinks he has a method to detect these strange particles by analysing the light from the Sun. His theory goes a little like this: some X-ray photons generated in the solar core (at a temperature of around 10 million Kelvin) will experience the Sun’s magnetic field, turning into axions. As the axions are weakly interacting, they will slip straight through the core and to the solar surface as if there was nothing there. Usually, photons would undergo countless absorption/emission events through the solar body, eventually escaping hundreds of thousands of years later (the path of a solar photon from core to surface is commonly referred to as the “drunkards walk”). The axions will essentially take a shortcut.
As the axion energy has not been lost through interactions with any solar matter, it has pretty much the same energy it had when it was generated seconds earlier. As the axion encounters the powerful magnetism in the solar corona (the Sun’s atmosphere) it could change again, but this time into an X-ray photon. This is where Hudson hopes to detect the signature of this strange particle by using existing data gathered by solar observatories such as Yohkoh, the High Energy Solar Spectroscopic Imager (RHESSI) and Hinode. None have been detected so far, but the Berkeley group are working hard to separate axion signature from solar “noise”.
Looking for the signature of axions generated in the Sun doesn’t stop with space-based observatories. The CERN Axion Solar Telescope in Geneva has its own superconducting magnet to artificially change any intact axions that have travelled all the way to Earth back into an X-ray photon.
If that wasn’t extreme enough for you, astrophysicist Karl van Bibber and his team at the Axion Dark Matter Experiment, Lawrence Livermore National Laboratory in California, wants to create their own axions by generating a powerful magnetic field and then looking out for the microwave signal after an axion decays into a single, real photon. Bibber’s explanation for the process is sheer quantum joy:
“The axion is so light that it doesn’t decay into two photons in free space. However, you can play a very remarkable trick. If I shoot a photon into a magnetic field (which you can think of as a sea of virtual photons), a real photon and a virtual photon [interact] to make an axion and vice versa.” – Karl van Bibber
Whether astrophysicists actually discover these ghostly particles is anyone’s guess, but it would seem that the axion hunt is beginning to catch up with all those WIMP detectors out there. Perhaps axions will be as common as neutrinos in a decade or so…
Original source: Space.com