Uniting Dark Matter and Antimatter to answer science’s most fundamental questions

Two of the most pressing mysteries in science are the nature of dark matter and the reason matter dominates over antimatter and a pioneering new experiment could hold the key to solving both.

Investigating how dark matter interacts with antimatter, and if this differs from how it interacts with ordinary matter could answer two of the most fundamental questions in science. What is dark matter, and why does matter vastly outweigh antimatter?

Enter scientists from the BASE collaboration and their pioneering new experiment to discover a discrepancy in the way antimatter interacts with dark matter as compared to everyday matter. Should this be the case, searching for dark matter with antimatter could reveal how the latter came to be dominated by its conventional matter counter-part.

“We’re looking for hints,” says Stefan Ulmer, spokesperson of the BASE collaboration, explaining the collaboration’s mission. “If we find a slight difference between matter and antimatter particles, it won’t tell us why the universe is made of matter and not antimatter, but it would be an important clue.”

Christian Smorra, co-team leader at BASE-Mainz (RIKEN)and first author of a paper published in Nature detailing the experiment continues: “The general assumption that dark matter behaves much like ordinary particles, meaning the interaction strength of dark matter with particles and antiparticles is assumed to be equal.

“This assumption has never been tested up until now.”

The BASE experiment is the first that attempts to search for dark matter with a particle of antimatter — in this case, a positron (RIKEN/Stefan Ulmer)

“Our study is the first atomic physics experiment searching for dark matter with antiparticles,” Smorra continues.

The mass range of dark matter candidates is extremely wide, Smorra explains, but this experiment offers a particular advantage in the search for dark matter. That is if it happens to be comprised of very light particles.

“There really are a huge variety of dark matter experiments ongoing at the moment, which reflects the large range of possibilities of candidate dark matter particles,” says Smorra. “Our experiment searches for very light particles, it’s most sensitive in a mass range 10²³ times lighter than an electron.”

This kind of lab-based tests also has advantages over astrophysical observations in the search for dark matter, Smorra tells me.

The answer is blowing in the wind

For their experiment, the BASE collaboration team used a specially designed device known as a Penning trap to ensnare a single antiproton. This prevents the anti-particle equivalent of the proton from making contact with ordinary matter and thus annihilating.

Penning trap system of the BASE Collaboration

Foto/©: Stefan Sellner, Fundamental Symmetries Laboratory, RIKEN, Japan

The particle was produced by scientists using the Antiproton Decelerator (AD) at CERN — currently the only research institution capable of generating low-energy antiprotons.

“We considered the antiproton’s ‘spin’ which is like a tiny compass needle attached to the particle,” Smorra explains. “Our experiment looks at the spin precession frequency of the antiproton — how many times per second the spin circulates around the magnetic field lines.”

This precession frequency should be constant in a stable magnetic field, therefore, a change in this frequency could be a result of an effect caused a particular candidate for dark matter — ultralight axion-like particles. In this particular model of dark matter the interaction the team looked for is called ‘axion wind.’

“The ‘wind’ comes from the velocity of the dark matter to relative to Earth, about 0.1% the speed of light,” Smorra says. “This axion wind potentially interacts with the spin of fermions [particles such as electrons, protons and their respective antiparticles] with a strength proportional to its velocity.”

A disturbance in the spin precession of the antiproton could indicate the effect of the axion wind (RIKEN/Stefan Ulmer)

As mentioned above, this axion wind effect has been searched for with conventional particles before, but this is the first experiment to look for it in the behaviour of an antiparticle.

The experiment is impressive given the level of precision the team achieved and the length of time over which they achieved it. “We can trap a single atomic nucleus in a particle trap, keep it there for more than a year, and control it very precisely” Smorra enthuses. “These kind of possibilities of controlling such a tiny object so precisely are, for me, fascinating.

“In addition, trapping antimatter is especially exciting, because it is such an exotic species in our world, and it takes a lot of care to keep it alive and protect it from collisions with matter.”

Future investigations and tackling the matter/ antimatter disparity

The experiment revealed no significant effect in the range of frequencies the team investigated. But this null-result has only inspired Smorra and the BASE team to improve their experiment so they may probe other frequencies in the future.

“We are already working on an improved experiment to make a more sensitive search for dark matter coupling or other sources of symmetry breaking when comparing protons and antiprotons,” the researcher explains. “New cooling methods will make our measurements more precise by taking them at extremely low temperatures — about 0.01 degrees above absolute zero.

“With these, we will be able to make a 10 to 100-fold more sensitive measurement in the next few years.”

Stefan Ulmer working at the BASE experiment at CERN's Antiproton Decelerator (AD).

Foto/©: Maximilien Brice / CERN

Smorra and the team also hope that their work will inspire other teams to attempt to replicate their experiment. “We hope that our study inspires other groups to consider investigating the coupling of other antiparticles such as positrons or anti-muons to axions,” He explains. “Such measurements could be performed in a similar way to what we have done in our study.”

And ultimately, an experiment of this kind could be the first step in explaining why there is an imbalance between matter and antimatter in the Universe. The Universe around us contains an abundance of matter in comparison to antimatter, and there is a good reason to believe that this disparity dates back to the earliest moments of the Universe, immediately after the big bang.

This is because processes that ‘create’ matter always create it in an equal amount to antimatter. The Big Bang — or any initial matter creation event — should be no different than events that create matter/ antimatter such as natural events like lightning, in that it should have created equal amounts of matter and antimatter — not the 9 to 1 split we observe today.

This poses a major problem for theories that seek to explain the formation of structure in the Universe. As particles of antimatter and matter annihilate on contact releasing their mutual energy back to the Universe — one should expect these early particles to have destroyed each other. As this happens extremely quickly, it is unlikely to leave matter around long enough to allow for the formation of the first atoms — not to mention stars, galaxies, planets and, eventually, us.

This means there must have been some early element of the Universe and its conditions that ‘favoured’ matter over antimatter. Thus far, the nature of this element has evaded scientists. Therefore, the investigation of differences in the behaviour of matter particles and antimatter particles are vital in beginning to tackle the question of disparity.

“These questions concern really a lot of scientists in different fields — astrophysics, particle physics and high-precision atomic physics,” Smorra explains. “These fields are related when we try to understand the composition of the Cosmos. After all, we have only identified 5% of the energy content of our universe.”

He points out another important fundamental question remains to be explained too. Namely, what is dark energy?

Smorra concludes: “If we have answered these questions, we will have made a big step forward in the understanding of our universe and the fundamental interactions in particle physics.”