Physicists to search for traces of dark matter in new experiment
No one knows what dark matter is made of, but it could be some kind of elementary particles. A candidate is the superlight axion that SDU physicist Manuel Meyer and his colleagues now hope to capture in an underground experiment.
Axion is the name of a hypothetical elementary particle. It is also the name of a washing detergent.
Physicist Frank Wilczek came up with the name axion in 1978 because the new hypothetical elementary particle would "clean up" some of the physicists' theories that stubbornly wouldn't quite fit together.
Since then, no one has been able to detect the axion. Still, many physicists hope that these particles exist because, as mentioned, they would "clean up" a problem in the theory that describes how atomic nuclei are held together and perhaps solve the puzzle of what dark matter is made of.
Dark matter constitutes about 80% of all matter in the universe, while the remaining 20% is the matter we know in the form of planets, galaxies, and everything we have on Earth, both natural and human-made.
We cannot feel or detect dark matter in the same way as known matter. For example, dark matter is probably constantly passing through our bodies without us noticing it. Therefore, physicists have to devise advanced experiments in their attempts to detect dark matter or one of the particles that dark matter may be made of.
If they ever succeed, it will be a revolution in physics.
Manuel Meyer, an associate professor at SDU, is one physicist pursuing dark matter. He is also the recipient of a European Research Council Starting Grant of 11 million DKK supporting his research: Searching for axion and axion-like-particle dark matter in the laboratory and with high-energy astrophysical observation.
Manuel Meyer is particularly interested in axions, the hypothetical elementary particles that would make so much sense if they are detected.
Now he and a number of other researchers from international research institutions are ready for a new major experiment in the hunt for axions: It is called the ALPS II experiment, and it will send photons (light particles) through a 240-meter-long tube for several weeks to months.
The special thing is that there is a wall set up in the tube.
"What we believe and hope for is that a photon will pass through the wall. It will not pass through as an ordinary photon - light cannot pass through a wall - but we have made a setup that allows the photon to transform into an axion, which can pass through the wall, and then transform back into a photon on the other side. In other words, if we see a photon on the other side of the wall - and it is not due to a measurement error - then we have likely had an axion pass through the wall", explains Manuel Meyer.
The superconducting magnets installed in the tube make it possible for a photon to be converted into an axion. After the magnets, there is the wall, which an axion will presumably be able to pass through.
Even though the researchers put a lot of effort into enhancing the chances for a magical shift from photon to axion and back – by using powerful laser beams, mirrors, and resonators - the chances are still small. Just as small as throwing 33 dice and having them all come up the same.
"If we see a photon behind the wall, our first thought would probably be that it's an error. But if it's not an error, it would be a revolution. We would have detected a new particle that can transform into a photon in a magnetic field. However, it doesn't mean it's a dark matter particle. While we can determine how strongly it interacts with a photon, we still wouldn't know its mass, and we still wouldn't know how much dark matter is made up from the axion", says Manuel Meyer.
His role in the ALPS II experiment is to look for photons on the other side of the wall. For that purpose, the researchers have a special photon-sensitive chip inside a cryostat, where the temperature is close to absolute zero (-273 degrees Celsius).
The chip is made of tungsten, which at this low temperature has no electrical resistance but becomes superconducting, requiring very little energy to heat it up. Actually, just a single photon is enough, and it will be detected by Manuel Meyer and his colleagues who monitor the chip's temperature.
"One can consider it as a highly sensitive thermometer that allows us to receive a signal from a photon and nothing else", he explains.
ALPS II is tentatively scheduled to start late May or early June 2023 with tests, followed by full run later in the year. It will run at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg.
Other than researchers from DESY, there will be participants from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), the Institute for Gravitational Physics at Leibniz University in Hanover, Cardiff University (UK), the University of Florida (Gainesville, Florida, USA), the Johannes Gutenberg University in Mainz, the University of Hamburg and the University of Southern Denmark.
Meet the researcher
Manuel Meyer is an associate professor and research leader at the Department of Physics, Chemistry, and Pharmacy.
Who else is looking for dark matter?
XENON is searching for heavy dark matter particles in a tank filled with liquid xenon.
CAST uses a strong magnet and optics from X-ray telescopes to search for axions from the Sun.
ADMX uses magnetic fields to attempt to convert dark matter axions into microwave photons.
Fermi LAT could indirectly detect dark matter through gamma rays from the annihilation of heavy dark matter particles or traces of photon-axion conversions in signals from distant galaxies.