'It's back to the drawing board...' Innocent axions found not guilty of dark matter crimes
Boffins look on the bright side of detecting basically nothing
Physicists have ruled out the existence of axions once considered possible dark matter candidates.
Dark matter has scientists stumped. It is widely believed that about 27 per cent of the universe is dark matter: mysterious globs of undetectable stuff that nobody really understands. Astronomers can observe its effects, and learn more about it through experiments and simulations, but precisely what dark matter is, well, it's not yet clear.
A team of physicists has now conducted a search for dark matter using laboratory equipment, rather than astronomical observations, and unfortunately found no sign of axions within a wide range of masses, which were believed to be what makes up the baffling invisible material. The results were analyzed and published in Physical Review X on Wednesday.
The experiment was carried out by researchers at the Paul Scherrer Institute in Switzerland, and the Laue-Langevin Institute in Grenoble, France, as part of the Neutron Electric Dipole Moment Collaboration.
The boffins trapped neutrons in a chamber using an electric and magnetic field. Although neutrons have no overall charge, they are made up of quarks that do individually have a positive or negative charge, and this gives neutrons a magnetic moment.
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In the presence of an external magnetic field, the neutron precesses with a frequency known as the Larmor frequency, due to that aforementioned magnetic moment. By switching the direction of the field, the scientists measured the change in a neutron’s Larmor frequency, and from that calculated its electric dipole moment, which is a measure of the distribution of positive and negative charge inside the neutron.
If axions were there as invisible dark matter, the extra presence of stuff would distort the readings in an observable manner. But the team detected no unexpected changes. There were no mystery guests to be found; no sign of the dark matter culprits.
Phillip Harris, coauthor of the paper and a professor of mathematical and physical sciences at the University of Sussex, England, said: "We've analysed the measurements we took in France and Switzerland and they provide evidence that axions – at least the kind that would have been observable in the experiment – do not exist.
“When we're searching for axions, we watch for whether the measurement fluctuates over time with a constant frequency. If so, it would be proof that there had been some interaction between the neutron and the axion. We never saw that."
There was no signal confirming the presence of axions between the wide mass range of 10−24 electronvolts and 10-17 electronvolts, essentially. While this does not rule out the possibility of the existence of axions, it just means the scientists have narrowed down their pursuit of dark matter candidates, said Nicholas Ayres, coauthor of the paper and a graduate physics student at Sussex uni.
"These results open a new front in the hunt for dark matter," he said. "They disprove the existence of axions with a wide range of masses and therefore help to limit the variety of particles which could be candidates for dark matter. And it's fantastic to see that these results – which were being collected for another purpose entirely – could be used as a piggyback to search for axions too."
Identifying dark matter will not only be a triumph for theoretical physics and a feat of engineering, it may help researchers understand why there is an imbalance of matter and antimatter in the universe.
At some point during the genesis of the universe, there would have been equal amounts of matter and antimatter, and they should have annihilated one another. Matter appears to have won that battle, as it's spread out all over the joint, whereas there's little or no antimatter left over. Dark matter is believed to unlock that strange asymmetry.
Before then, however, these results "essentially send physicists back to the drawing board in our hunt for dark matter," Harris concluded. ®
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