Laser cooling of antimatter particles could rewrite modern physics

March 31, 2021

Scientists at CERN are close to "freezing" antimatter particles for study. (AP Photo/Anja Niedringhaus)

One of the few teams in the world with access to antimatter has found a way to examine these mysterious particles by cooling them to near absolute zero, bringing scientists close to finally understanding their elusive behavior and answering some of the most pressing questions in modern physics.

This groundbreaking use of laser cooling to study antimatter particles by bringing them to rest was detailed in a paper published March 31 in Nature

It's easiest to understand particles when they are at rest because if a particle is excited, it bounces around, reacts with things or completely blows up. In experiments conducted at the European Council for Nuclear Research, the scientists applied this notion to antimatter particles. 

"ALPHA is an experiment that can produce and trap antihydrogen atoms," explained Jeffrey Hangst, the study's lead and a physicist at Aarhaus University in Denmark. "They don't exist naturally; that's why we work at CERN. We need the decelerator to produce the antiprotons."

The researchers aim to identify if physics theorists have been correct so far, as most assume antimatter behaves symmetrically, just like normal matter particles. Stating that every reaction has an equal and opposite reaction, Newton's Third Law alludes to the natural parallels found in the universe, and antimatter is often thought to be no exception.

"In the history of physics, we've thought that types of symmetry were exact, and been wrong," Hangst said. "This is a hard one to get around — what we're looking for now — but there's historical precedents for these things to change."

For example, the parity phenomenon can be summarized as the difference between left and right. Scientists previously speculated that if the universe was mirrored, nothing would fundamentally change.

"We used to think that nature doesn't know the difference between left and right. But actually, there are some interactions that maximally violate that — they go only to the left," Hangst said. "Each time that we find a violation of these [symmetries], somebody wins a Nobel Prize. It's a fundamental change in our understanding when you violate the symmetry."

In the realm of modern physics, the behavior of the tiniest possible particles in the universe, such as quarks, which are even smaller than protons and electrons, are described by something called the Standard Model. Based on discoveries since the 1930s, it explains how these particles interact with each other, their generations, forces and movements, and iterates many other qualities of the components of regular matter. 

Normal matter makes up everything in the observable universe, from flowers outside to people's bodies, and even stellar objects like the sun. However, the widely accepted Standard Model indicates something about matter that has continued to elude scientists.

"It predicts for our system hydrogen and antihydrogen — and that they must behave identically," Hangst said.

The concept of antimatter, in this case antihydrogen, can be described by thinking about square roots. The square root of four can be positive two, but also negative two. Both integers provide the same answer, but have opposing charges. 

Antimatter atoms are exactly like regular matter atoms, which contain protons and electrons, except every charge within the atom is the opposite. Protons are negative, and electrons are positive.

"Each type of fundamental particle we've identified has this opposite. The trick is that they can't coexist," Hangst said. But, "These things both exist. For some reason, the universe is made up of one of them, and we call that normal matter. We actually don't know why that is."

When the Big Bang occurred approximately 14 billion years ago, there was a huge release of energy. That energy created stars and galaxies, which appear to be made of normal matter. But, returning to the square root analogy, there should have been antimatter comprising that energy, too.

Yet where that antimatter went when this energy divided is one of today's biggest unanswered questions in physics.

Setting out to find an answer, Hangst and his team at CERN sought to cool the antimatter particles they generated to absolute zero — the closest possible state to complete rest.

The method they used, called laser cooling, is ubiquitous in the physics community.

"Practically, for 30 years, people have been cooling atoms and ions in the university lab — you can do this as a tabletop experiment," Hangst said. "And there have been several Nobel Prizes associated with work like this."

However, he noted, "This is the first time for antimatter, and we're the only ones who have this capability and are likely to have it anytime soon."

Not only are the researchers pioneering the mechanism for antimatter, they're also overcoming a steep hurdle that antimatter particles pose for cooling. 

Because these particles are synthesized, they're kind of in the wrong universe. The lab's environment has normal matter, what humans are used to, so the antimatter particle is an intruder.

"Hydrogen is easy to cool; it doesn't annihilate when it comes into contact with things," Hangst explained. "So you can bounce it off of cold surfaces and it cools down. But with anti-matter, that's obviously not an option. We have to stay in a very, very good vacuum; it can't come in contact with any other matter."

To cool an atom is to slow it down, because there's a one-to-one relationship between temperature and energy at the atomic level. Normally, atoms are excited and jittery, but when they're interrupted by a force, such as a laser pulse, they recoil and slow their velocity.

"It's a really practical issue of, 'How well can you measure something?'" Hangst asked. "How well can you interact with it? Both of the things that we're concentrating on benefit greatly from this ability to slow them down."

The team's current objective is two-fold: figure out what kind of light these particles absorb and emit, and dissect their gravitational behavior.

"There's no good theory that says that we're going to find something — I have to be very clear about that," Hangst acknowledged. "The laws of physics as we understand them look pretty good, and they say that our experiments shouldn't find a difference." 

"That means you have to look very carefully. Maybe you don't find something," he continued. "But if you do, it's revolutionary — and everything changes."

The paper, "Laser cooling of antihydrogen atoms," was published March 31 in Nature. It was authored by C. J. Baker, M. Charlton, A. Cridland Mathad, S. Eriksson, C. A. Isaac, J. M. Jones, P. Knapp, N. Madsen, D. Maxwell, J. Peszka, P. S. Mullan, A. Powell and D. P. van der Werf, Swansea University; W. Bertsche, D. Hodgkinson, M. A. Johnson and M. Sameed, University of Manchester; A. Capra, R. Collister, P. Grandemange, D. R. Gill, M. C. Fujiwara, L. Kurchaninov, A. Khramov, J. T. K. McKenna, J. M. Michan, T. Momose, A. Olin, D. M. Silveira, K. Olchanski, C. So, R. I. Thompson and A. Thibeault, TRIUMF; C. Carruth, A. Christensen, J. Fajans, E. Hunter and J. S. Wurtele, University of California at Berkeley; C. L. Cesar and R. L. Sacramento, Universidade Federal do Rio de Janeiro; T. Friesen and A. Evans, University of Calgary; N. Evetts and W. N. Hardy, University of British Columbia; P. Granum, J. S. Hangst, S. A. Jones and G. Stutter, Aarhus University; M. E. Hayden and J. J. Munich, Simon Fraser University; S. Jonsell, Stockholm University; S. Menary and D. M. Starko, York University; P. Pusa, University of Liverpool; C. Ø. Rasmussen, CERN; F. Robicheaux, Purdue University; E. Sarid, Purdue University; and T. D. Tharp, Marquette University.

Correction: A previous version of this article incorrectly described the development of the Standard Model. The error has been corrected.

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