Jul 3 2019
Humans are surrounded by matter day and night in every form—houses, furniture, trees, and even the air inhaled.
Dr Teng Wu aligning the probe laser beam of the comagnetometer setup. (Image credit: Arne Wickenbrock)
However, according to physicists, the visible matter known to mankind may only constitute around 20% of all material in the universe. According to the existing theory, approximately 80% may be dark matter. This claim is on the basis of a number of observations, one of which is that if only ‘normal’ matter exists in the universe, then stars and galaxies will rotate much faster than they would.
Dark Matter Could be Made of Axions
Eventually, researchers have formed various theories to describe what exactly this puzzling dark matter might be composed of. Weakly interacting massive particles, or WIMPs for short, are one of the promising candidates that come into question. Scientists have spent several years attempting to find these with particle detectors, but so far without any success.
However, many years ago, researchers projected an alternative—a class of particles known as axions, which are considerably lighter in comparison to other particles. The theory states that the field of these particles oscillates, which implies that it varies continuously. This oscillation’s frequency is proportional to the particles’ mass, and, since this is very low, the frequency should also be low. However, nobody knows until now whether that is the case. The issue is that the field oscillation is as likely to undergo a complete cycle every year as a trillion times a second.
Detecting Axions with the Help of Nuclear Spin Change
Scientists at Johannes Gutenberg University Mainz (JGU) have presently identified a method for detecting axions using the Cosmic Axion Spin Precession Experiment (CASPEr) program.
We are exploiting the potential of nuclear magnetic resonance. This means we can identify the spin of nuclei within molecules, or, more specifically in our case, within the carbon isotope C13 and hydrogen.
Professor Dmitry Budker, Institute of Physics, JGU and Helmholtz Institute Mainz
The fundamental theory is that dark matter can have an impact on the spin of nuclei, thus offering scientists a way of hunting it down. However, the spin can also be affected by the magnetic field of the earth. Although the scientists employed advanced shielding to curb the magnetic field, even the best shielding was found to be imperfect. Hence, the physicists should determine which proportion of the observed spin changes is caused by dark matter and which proportion is caused by the Earth’s magnetic field. This made the research group to create its new comagnetometer configuration. The principle behind the method is the fact that molecules usually consist of various types of atomic nuclei. Since different nuclei will react to the magnetic field and dark matter to varying extents, these impacts can possibly be differentiated.
A Part of the Possible Frequency Range has now been Investigated
The research group at Mainz University has presently gone through the range of frequencies from a few oscillations per year up to 18 oscillations per hour—up to now, without finding proof of the influence of dark matter. “It’s rather like looking for a lost ring in a vast garden,” said Budker. “We have already searched part of the garden, so we now know this is where the ring—the axion—is not to be found. This has allowed us to considerably narrow down the range in which we hope to find the axion, and we can now focus our search on other ranges.”