The hunt for axions – a potential dark matter candidate – at the CERN Axion Solar Telescope has been fruitless. But scientists refuse to give up as they set a new limit that calculates the probability of finding these elusive particles.
Axion particles are suspected to emerge from stars. Sunlight is filtered through the CERN Axion Telescope (CAST) for 90 minutes a day during dusk and dawn, and is directed onto a magnet originally used at the Large Hadron Collider.
The idea is that the strong magnetic field will cause the axions to convert to X-ray photons, which can be measured by X-ray detectors.
Schematic of how CAST works. The two dashed lines mark the pipes in the magnet where axions are converted to photons (Image credit: CAST collaboration and Nature Physics)
CAST has been updated repeatedly to accommodate for new physical constraints to narrow down the search for axions. After combing through the data taken from the 2013-2015 CAST run, a large international group of physicists working under the CAST collaboration have moved the posts again.
They have recalculated the axion-photon “coupling strength” to 95 per cent confidence. This measures the strength of the interaction between axions and photons – or in other words the probability of axions turning into photons.
If you want to impress your friends down at the pub with numbers, a paper published this month in Nature Physics states the coupling strength to be 0.66 x 10-10 per giga-electron volt (GeV-1) – it’s an extremely weak interaction.
Last year, another study estimated that the mass of an axion particle was up to ten billion times lighter than an electron. Scientists reckon every cubic centimetre of the universe contains about 10 million of these super-lightweight particles on average. But the tiny mass and low interaction strength make them particularly difficult to find.
Over three quarters of the mass-energy in the universe is dark matter. Finding out what the mysterious stuff is made out of could explain a whole host of unsolved problems – such as how galaxies are held together, how they collide, and inconsistencies in the cosmic microwave background.
Axions aren’t the only contender for dark matter. Several other observatories, such as the Large Underground Xenon dark matter experiment (LUX), are looking for Weakly Interacting Massive Particles (WIMPs). Last year, the researchers there announced that they had no luck finding dark matter either. ®