Scientists cool positronium to near absolute zero for antimatter research

Most atoms are made up of positively charged protons, neutral neutrons, and negatively charged electrons. Positronium is an exotic atom composed of a single negative electron and a positively charged antimatter positron. Its lifetime is naturally very short, but researchers, including those at the University of Tokyo, have managed to cool and slow down samples of positronium using carefully tuned lasers.

The results are published in the journal NatureThey hope the research will help other researchers explore exotic forms of matter and could reveal the secrets of antimatter.

A part of our universe is missing. You may have heard such a bizarre statement if you have read much about cosmology in the last few decades. The reason scientists say this is because almost everything we see in the universe is made of matter, including you and the planet you are on.

However, we have long known about antimatter, which, as the name suggests, is sort of the opposite of ordinary matter, in that antimatter particles share the same mass and other properties as their matter counterparts, but have an opposite charge. When matter and antimatter particles collide, they annihilate each other, and it is widely believed that they were created in equal quantities at the dawn of time. But that is not what we see today.

“Modern physics only accounts for a portion of the total energy in the universe. Studying antimatter could help us explain this difference, and we have just taken a big step in this direction with our latest research,” said Associate Professor Kosuke Yoshioka of the Photon Science Center.

“We have succeeded in slowing down and cooling exotic atoms of positronium, which is 50% antimatter. This means that, for the first time, it is possible to explore this matter in a way that was previously impossible, which will necessarily involve a more in-depth study of antimatter.”

Positronium sounds like something out of science fiction, and despite its very short life span, it is a very real thing. Think of it as the familiar hydrogen atom, with its relatively large, positively charged central proton and its tiny, negatively charged electron orbiting it, except you replace the proton with the antimatter version of the electron, the positron.

This results in an exotic atom that is electrically neutral but lacks a large nucleus; instead, the electron and positron exist in a mutual orbit, making it a two-body system.

Hydrogen is also a many-body system, since a proton is actually made up of three smaller particles, called quarks, stuck together. And because positronium is a two-body system, it can be fully described by traditional mathematical and physical theories, making it ideal for testing predictions with extreme precision.

“For researchers like us, involved in what is called precision spectroscopy, being able to precisely examine the properties of cooled positronium means that we can compare them with precise theoretical calculations of its properties,” Yoshioka said.

“Positronium is one of the few atoms that consists of only two elementary particles, which allows for such precise calculations. The idea of ​​cooling positronium has been around for about 30 years, but an offhand comment from Kenji Shu, an undergraduate student who is now an assistant professor in my group, inspired me to take on the challenge of making it happen, and we finally succeeded.”

Yoshioka and his team had to overcome several challenges in cooling positronium. First, there is the problem of its short lifetime: one ten millionth of a second. Second, there is its extremely light mass. Because it is so light, it is impossible to use a cold physical surface or other substance to cool it. So the team resorted to lasers.

You might think that lasers are very hot, but in reality, they are just packets of light, and it is how the light is used that determines the physical impact it has on something. In this case, a weak, finely tuned laser gently nudges a positronium atom in the opposite direction of its motion, slowing it down and cooling it in the process.

By doing this repeatedly and in just one ten millionth of a second, portions of the positronium gas were cooled to about 1 degree above absolute zero (-273 degrees Celsius), the coldest temperature that can be achieved. Given that positronium gas is at 600 Kelvin, or 327 degrees Celsius, before it cools, this is quite a dramatic change in such a short time.

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

“This experiment only used a laser in one dimension. If we use all three, we will be able to measure the properties of positronium with even greater precision. These experiments will be important because we may be able to study the effect of gravity on antimatter. If antimatter behaves differently from ordinary matter because of gravity, it could help explain why part of our universe is missing.”