Achieving Breakthrough in Positronium Research

Researchers, including those from the University of Tokyo, used precisely calibrated lasers to cool and slow down samples of positronium in a study that was published in Nature.

Positively charged protons, neutral neutrons, and negatively charged electrons make up the majority of atoms. A single negative electron and a positively charged antimatter positron make up the unusual atom known as positronium.

Researchers believe that by assisting others in their exploration of unusual forms of matter, their study could help reveal the mysteries of antimatter.

The universe is missing a portion. If you have read anything about cosmology in the past few decades, you may have heard such an odd assertion. Scientists believe this because nearly everything in the cosmos, including you and the planet you're standing on, is composed of matter.

However, as the name implies, antimatter is essentially the opposite of conventional matter in that its particles have the opposite charge but the same mass and other characteristics as their matter counterparts. This is something we have known about for quite some time. It is generally accepted that matter and antimatter were formed in equal quantities at the beginning of time since they annihilate when they encounter. But that is not what is now observed.

Modern physics only accounts for a part of the total energy of the universe. The study of antimatter might help us account for this discrepancy, and we’ve just taken a big step in this direction with our latest research. We have successfully slowed and cooled down exotic atoms of positronium, which is 50% antimatter. This means that for the first time, it can be explored in ways previously impossible, and that will necessarily include a deeper study of antimatter.

Kosuke Yoshioka, Associate Professor, Photon Science Center, University of Tokyo

Positronium might sound like something from science fiction, yet it is a genuine material despite having a relatively short lifespan. Imagine it as the well-known atom hydrogen, with a small, negatively charged electron in orbit and a central, positively charged proton that is quite massive. However, instead of the proton, you would have the positron, which is the antimatter equivalent of the electron.

This results in an exotic atom that is a two-body system since the electron and positron are in mutual orbit. The atom is electrically neutral but lacks a substantial nucleus. Since a proton is actually three smaller particles called quarks locked together, even hydrogen is a multibody system. Furthermore, because positronium is a two-body system, it can be fully explained by conventional mathematical and physical theories, which makes it perfect for extremely accurate prediction testing.

Yoshioka added, “For researchers like us, involved in what is called precision spectroscopy, being able to examine the properties of cooled positronium precisely means we can compare them with precise theoretical calculations of its properties. Positronium is one of the few atoms made up entirely of only two elementary particles, which allows for such exact calculations. The idea of cooling positronium has been around for around 30 years, but a casual comment by undergraduate student Kenji Shu, who is now an assistant professor in my group, prompted me to take on the challenge of achieving it, and we finally did.”

Yoshioka and colleagues faced several challenges in their quest to cool positronium. The first problem is its brief lifespan of one ten millionth of a second. Second, it has an extremely low mass. The scientists employed lasers to chill positronium since it is so light that they could not utilize a cold surface or any other material. Although lasers are thought to be extremely hot, in reality, they are only light packets, and the physical effects of light are dependent on how they are employed.

Here, a positronium atom is gently pushed in the opposite direction of its motion by a weak, precisely calibrated laser, which slows and cools the atom. It was possible to cool parts of positronium gas down to around 1 degree above absolute zero (-273 degrees Celsius) by repeatedly doing this in as little as a ten-millionth of a second. Considering that positronium gas cools to 600 kelvins, or 327 degrees Celsius, before becoming stable, this is a significant shift in a short amount of time.

“Our computer simulations based on theoretical models suggest that the positronium gas might be even colder than we can currently measure in our experiments. This implies that our unique cooling laser is very effective at reducing the temperature of positronium and the concepts can hopefully help researchers study other exotic atoms,” stated Yoshioka.

Yoshioka concluded, “This experiment used a laser in just one dimension, however, and if we utilize all three, we can measure the properties of positronium even more precisely. These experiments will be significant because we may be able to study the effect of gravity on antimatter. If antimatter behaves differently to regular matter due to gravity, it could help explain why some of our universe is missing.”

Journal Reference:

Shu, K., et al. (2024) Cooling positronium to ultralow velocities with a chirped laser pulse train. Nature. doi.org/10.1038/s41586-024-07912-0