Scientists searching for the elusive dark matter, which appears to make up 80 percent of the matter in the universe, have managed to narrow down the range of possible suspects. Researchers at the University of Sussex have disproved the existence of certain kinds of axions, particles that are a leading candidate for dark matter, and while it may send physicists back to the drawing board, the hunt can be more focused in future.
Dark matter is a tricky substance to pin down. Since it doesn't interact with electromagnetic radiation, it doesn't reflect light at all and can't be directly observed, but its gravitational effects can still be felt. The movements of stars and galaxies don't make sense based on visible matter alone, leading astronomers in the 1930s to hypothesize that some unseen mass was at play. And scientists have been searching for it ever since.
The list of likely suspects has shrunk over the years. Weakly Interacting Massive Particles (WIMPs) with certain masses have been ruled out by several runs of experiments using the Large Underground Xenon (LUX) detector, and the HADES particle detector determined that dark matter wasn't composed of "dark photons."
Axions however remained a strong candidate. Scientists are trying to not only determine if these hypothetical elementary particles exist, but also what their masses might be. The new study has looked at data gathered by the Neutron Electric Dipole Moment (nEDM) experiment and returned a null result, narrowing down the range of masses that axions could have.
"If axions with the right properties exist it would be possible to detect their presence through this entirely novel analysis of our data," says Philip Harris, head of the nEDM group at the University of Sussex. "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. These results are a thousand times more sensitive than previous ones and they are based on laboratory measurements rather than astronomical observations."
In the nEDM experiment, neutrons are trapped in specially designed containers, which are then electrified. The aim is to check whether the high voltage affects the rate at which the neutrons spin, and if that frequency changes over time, it would indicate the presence of axions. Since no such distortions were detected, that means there were no axions within the mass range that the instrument can pick up.
The nEDM experiment was originally run to solve a different cosmological mystery. In the beginning of the Universe, matter and antimatter should have been created in equal amounts, but today matter is common while antimatter is almost non-existent. The experiment was designed to study how this asymmetry came to pass, but the Sussex researchers realized that looking at the data in a different way could reveal the existence of axions.
"In our original experiment we took a single measurement and repeated it many times to determine the average value over a long time," says Harris. "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."
Although it returned a null result, the scientists point out that axions could still exist, just not with the properties that the nEDM experiment tested for. They might not have been interacting with the neutrons strongly enough, or they may have masses that are larger or smaller than the expected range. Future work will have to look elsewhere.
"This does not fundamentally rule out the existence of axions, but the scope of characteristics that these particles could have is now distinctly limited," says Harris. "The results essentially send physicists back to the drawing board in our hunt for dark matter."
The research was published in the journal Physical Review X.
Source: University of Sussex
