Positronium

Two tiny parts join together.

One part is an electron. The other part is a positron. They dance around each other like a tiny atom. But they do not stay together for long. Soon, they go pop! This is called annihilation. When they pop, they turn into bright light. This light can come in two or three pieces. It is a very fast and tiny event. Can you imagine tiny things dancing and then turning into light?

Positronium is a very strange kind of atom.

It is made of two tiny parts. One part is an electron. The other part is a positron. A positron is the anti-particle of an electron. They stay bound together in a tiny system. This system is called an onium. Unlike a normal hydrogen atom, it has no proton.

Positronium does not last long. The two parts will eventually annihilate each other. This means they destroy each other. When they do, they turn into light called gamma rays. This happens in different ways. If the parts have opposite spins, it is called para-positronium. This type usually makes two gamma rays. If the spins are the same, it is called ortho-positronium. This type usually makes three gamma rays. Scientists use these tiny events to test their ideas about physics. They can even use lasers to cool it down.

Caption: An electron and positron orbit each other. Caption: A lab used to study positronium.

Positronium is a very strange and exotic type of atom.

It is called an onium because it is a bound system of two particles. Most atoms, like hydrogen, have a proton at their center. However, positronium has no proton at all. Instead, it is made of an electron and its anti-particle, a positron. These two tiny particles stay bound together into a single system. This unique structure makes it a very important subject for scientists to study.

This tiny system is unstable and does not last very long. The electron and the positron will eventually annihilate each other. This means the two particles destroy one another. When they annihilate, they turn into energy in the form of gamma rays. The way they destroy each other depends on their spin states. Spin is a property that tells us how the particles are oriented. If the spins are opposite, it is called para-positronium. This type usually produces two gamma rays.

Other types of positronium exist based on how the particles spin. The triplet states have parallel spins and are called ortho-positronium. This type usually decays into three gamma rays. Para-positronium has a very short mean lifetime of about 0.12 nanoseconds. Ortho-positronium lasts a bit longer, with a lifetime of about 142 nanoseconds in a vacuum. These different ways of decaying allow scientists to perform precision tests. They use these measurements to check the rules of quantum electrodynamics.

Humans have been studying this since the mid-1900s. The Croatian physicist Stjepan Mohorovičić predicted it in 1934. He originally called the substance "electrum." Later, Martin Deutsch experimentally discovered it at MIT in 1951. Since then, many experiments have verified the math behind how it works. In 2024, the AEgIS collaboration at CERN reached a new milestone. They were the first to cool positronium using laser light.

Positronium is quite different from the hydrogen atoms we see in nature. In a hydrogen atom, the proton is much heavier than the electron. In positronium, the electron and positron have the same mass. This makes the energy levels of positronium about half of those in hydrogen. Scientists can even create molecules using these particles. For example, they have made molecules of positronium hydride. They have also observed di-positronium, which is made of two positronium atoms.

{ "text": "Positronium (Ps) is an exotic, unstable system made of two particles. It consists of an electron and its anti-particle, a positron, bound together.

This system is classified as an onium. Most atoms, such as hydrogen, are held together by a heavy central nucleus. However, positronium has no proton at its center. Instead, the electron and positron orbit a common center of mass. This unique structure makes it a vital tool for testing the laws of physics. Scientists use it to study the behavior of matter and antimatter.\n\nThe mechanism of positronium involves a process called annihilation. Because the electron and positron are matter and antimatter, they cannot stay together forever. They eventually destroy each other and convert their mass into energy. This energy is released as gamma rays, which are high-energy light particles. The specific way they annihilate depends on their relative spin states. Spin is a fundamental property related to the particles' orientation. Depending on these states, the system may produce two or three gamma rays.

\n\nThere are two primary types of positronium based on these spin states. The first is para-positronium (p-Ps), which is the singlet state. In this state, the spins of the electron and positron are antiparallel, meaning they point in opposite directions. Para-positronium has a very short mean lifetime of about 0.12 nanoseconds in a vacuum. It decays mostly into two gamma rays, each with an energy of 511 keV. It can also decay into four or six photons, though this is very rare.

\n\nThe second type is ortho-positronium (o-Ps), known as the triplet state. In these states, the spins are parallel, meaning they point in the same direction. Ortho-positronium has an energy about 0.001 eV higher than the singlet state. It has a much longer mean lifetime of approximately 142 nanoseconds in a vacuum. The most common decay for this state produces three gamma rays. Because of its longer life, it is easier to observe in certain experiments.

\n\nThe history of positronium began with theoretical predictions. In 1934, Croatian physicist Stjepan Mohorovičić predicted its existence. He originally referred to the substance as \"electrum.\" It was not until 1951 that Martin Deutsch experimentally discovered it at MIT. Since that discovery, researchers have used positronium to perform precision tests of quantum electrodynamics (QED). QED is the theory that describes how light and matter interact. In 2024, the AEgIS collaboration at CERN achieved a milestone by cooling positronium using laser light.

\n\nPositronium behaves very differently from the hydrogen atom. In hydrogen, the proton is much heavier than the electron. In positronium, the electron and positron have equal masses. This equality affects the system's reduced mass, which is a value used in energy equations. Because of this difference, the energy levels of positronium are roughly half those of hydrogen. The lowest energy orbital state is called 1S. The mass of the entire positronium system is 1.022 MeV. This is equal to twice the electron mass minus the binding energy.

\n\nScientists have also discovered that positronium can form complex structures. They have created exotic compounds like positronium hydride (PsH). Researchers have also observed di-positronium molecules, which consist of two positronium atoms.

In materials, the behavior of positrons is quite complex. When a positron enters a material, it may form positronium before annihilating. About 60% of positrons annihilate directly with an electron. About 10% form para-positronium, while 30% form ortho-positronium that is quickly \"picked off\" by nearby electrons. Only about 0.5% of positrons form ortho-positronium that undergoes its natural, slow decay.

", "media": [ "File:Posit