Dark matter search advances with new experiment to spot axions

An experiment first devised 30 years ago is now commencing led by a team of international scientists to spot axions, mysterious particles linked to dark matter.
Paul Ratner
The ALPS experiment
  • Dark matter is theorized to make up more than 30 percent of the matter-energy composition of the universe, yet has proved hard to detect.
  • An experiment, first devised 30 years ago, has now commenced in Germany to try to spot axions.
  • Axions are theoretical particles that are linked to dark matter.

One of the great enigmas of the universe is the hypothetical existence of dark matter. The matter which we can see and interact with is predicted to make up just 5 percent of the matter in the universe, and only around 0.5 percent of the matter-energy composition of the universe, with dark energy and dark matter taking up the rest.

Although dark matter supposedly makes up about 30 percent of the universe, it has never actually been detected. Now, a new experiment will use the world’s most sensitive instrument of its kind is set to scour for axions – potential dark matter particles. 

According to the German research center Deutsches Elektronen-Synchrotron (DESY) in Hamburg, a photon can be theoretically converted into an axion or an axion-like particle (ALP) and vice versa inside a magnetic field.

These particles, collectively termed WISPs (Weakly Interacting Sub-eV Particles) have a very small mass and react very weakly with the remaining matter particles. They arise naturally in some versions of the Standard Model of physics and the researchers hope their presence (if proven) may explain observations that are not accounted for within the Standard Model.

To generate axions, the researchers devised a “light shining through a wall“ experiment called "Any Light Particle Search II" (ALPS II). This takes place at the particle accelerator housed at DESY.

This type of experiment uses a high-power laser propagating through a magnetic field to generate a beam of axion-like particles. These then travel through a wall which blocks the laser light. Due to the hypothetical axions' weak interaction, they pass through a wall. On the other side, the axions are re-converted into photons in another strong magnetic field.

The magnetic field is generated using 24 recycled superconducting magnets from the HERA particle accelerator, a powerful laser beam, extremely sensitive detectors, and precise interferometry to look for the elusive particles.

They are predicted to be very weak, interacting with matter we know in a way that’s nearly impossible to observe and detect. As such, first conditions need to be created that could allow us to spot them.

Dark matter search advances with new experiment to spot axions
Magnet row of the ALPS experiment in the HERA tunnel.

The experiment

The team behind the experiment is taking a novel approach that was first proposed over 30 years ago but was not possible to carry out till now. ALPS will attempt to use a strong magnetic field to transform photons, particles of light, into axions.

The process involves sending a high-intensity laser beam along a so-called "optical resonator device" into a vacuum tube that’s about 120 meters long. There the beam, enclosed by 12 HERA magnets, is reflected back and forth while specially-developed control electronics make sure the laser is perfectly tuned.

If this process succeeds in turning a photon into an axion or ALP within the strong magnetic field, the axion would go through an opaque wall that’s at the end of the line of magnets. 

If it gets through the wall, the axion would end up on another identical magnetic track, where it would turn back into a photon, which would then be caught by a detector. A second optical resonator set up at that point would work to increase the probability of the axion turning back into a photon by a factor of 10,000, as explained in a statement from the team.

Mysterious axions

Axions are elementary particles that have been hypothesized but have not yet been detected. They were first proposed by the theoretical physicist Roberto Peccei and his colleague Helen Quinn in 1977 as a way to solve a major physics conundrum — the so-called “strong CP problem."

The CP problem is a puzzle within the Standard Model of particle physics regarding the strong nuclear force. It binds quarks to create protons and neutrons within the nuclei of atoms and somehow makes the structure of the neutrons perfectly symmetrical.

In the Standard Model, a combination of charge conjugation (C), which replaces particles with antiparticles, and parity (P), which replaces particles with their mirror-image counterparts, are allowed in both weak and strong interactions. However, it has only ever been observed in the weak interaction. The fact that it's allowed in the strong interaction, but has never been seen, is the strong CP problem.

The possible existence of axions would answer the question of why the charge carried by the quarks appears to spread out very uniformly and why the CP combination has not been seen in strong interactions. 

The team

Besides DESY, the research involved an international collaboration of around 30 scientists from around the world, with participants from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), the Institute for Gravitational Physics at Leibniz Universität Hannover, the University of Florida in Gainesville, the Johannes Gutenberg University in Mainz, the University of Hamburg and the University of Southern Denmark (Odense).

The scientist’s perspective

Interesting Engineering (IE) reached out to DESY’s Axel Lindner, project leader and spokesperson of the ALPS collaboration, for more insight on the experiment.  

The following exchange has been lightly edited for clarity and flow.

Interesting Engineering: Why has dark matter been so hard to detect so far?

We know about the existence of dark matter in the universe only by the gravitational force acting on length scales way beyond the solar system. Therefore, dark matter constituents can interact at maximum only very weakly with "normal" matter. Thus our detectors made out of "normal" matter have a very hard time detecting dark matter. In fact, none of the numerous experiments have been successful up till now in spite of very impressive technological progression.

IE: What is different about this experiment from previous research into dark matter?

ALPS II targets a specific species of hypothetical dark matter candidate particles called axions. In contrast to many other experiments, we try to produce the axions in the first part of the experiment and detect it in the second compartment.

This way we don't have to rely on assumptions regarding the spatial distribution of the dark matter in our Milky Way, for instance. However, we have to pay a price – the model independence of ALPS II comes with a reduced sensitivity compared to experiments focusing on the dark matter in our home galaxy.

IE: If axions are detected by this research, what would it mean for our understanding of the fundamental forces of the universe?

A detection would demonstrate that a new class of elementary particles exists. This axion may just be the first of its kind, some theorists predict an "axiverse" consisting of many axions and axion-like particles. Clearly, the detected axion would be the prime candidate to explain the dark matter in the universe. Many follow-up experiments will surely follow.’

For example, searches for solar axions (the IAXO experiment) have the potential to measure details of axion production in the sun (especially, if we could fold in a positive detection by ALPS II), which in turn might even pave the way to physics beyond the standard model of particle physics: axions might indicate that string theory is the more fundamental model to describe nature.

Last but not least, extremely lightweight axions could mimic the dark energy in the universe.

IE: What other research are you planning to conduct using ALPS after the search for axions?

Axion searches will keep us busy for a couple of years. DESY is also strongly involved in the axion experiments IAXO and MADMAX.

After the axion searches, we might use ALPS II for the first measurement of the so-called vacuum magnetic birefringence predicted in the 1930s. We are also investigating options to search for high frequencies of gravitational waves with ALPS II.

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