In this artist's depiction of how experimentalists could create true muonium, an electron (blue) and a positron (red) collide on the left side of the image, producing a virtual photon (green) and then a muonium atom, made of a muon (small yellow) and an anti-muon (small purple). The muonium atom then decays back into a virtual photon and then a positron and an electron. Overlaying this process is a figure indicating the structure of the muonium atom: one muon (large yellow) and one anti-muon (large purple).

Terry Anderson, SLAC

A new window allowing scientists to peer a little deeper into the atomic depths of the universe may soon open, thanks to researchers who have devised a way to create and observe an elusive, exotic atom—predicted, but never seen—called "true muonium."

The existence of true muonium was first theorized more than 50 years ago, but since then no one had been able to come up with a method by which the elusive speck could be created and observed with certainty.

Now, Stanley Brodsky, a theoretical physicist at SLAC National Accelerator Laboratory, operated for the Department of Energy by Stanford, and Richard Lebed, a physicist at Arizona State University, have found not one, but two methods that should work. They won't know for sure until other researchers try out their recipes.

"We don't usually work in this area, but one day we were idly talking about how experimentalists could create exotic states of matter," said Brodsky. "As our conversation progressed, we realized, 'Gee, we just figured out how to make true muonium.'"

True muonium is made of two particles, a muon and an anti-muon. A muon is one of the elementary particles of matter and is similar to an electron in that it has a small negative charge. All elementary particles have a corresponding antiparticle with the opposite charge, which for a muon is the positively charged anti-muon. The name "true muonium" is used to distinguish the atom composed of a muon and anti-muon from a different atom that has been called "muonium," but consists of an electron and an anti-muon.

Both muons and anti-muons are created frequently in nature when energetic particles from space strike Earth's atmosphere. Yet both have a fleeting existence, and their combination, true muonium, decays naturally into other particles in a few trillionths of a second. This makes observation of the exotic atom quite difficult.

Physicists care about muons—and muonium—partly because studying elementary particles is a good way to better understand the laws of nature. And because muons are created by cosmic rays hitting our atmosphere, they can reveal information about cosmic rays, effectively making muons messengers from space.

Another intriguing aspect of muons is their ability to pass through other matter with relative ease. Showering down from the upper atmosphere, they go through plants, animals, buildings, cars and even people. Muons can go thousands of meters into the Earth itself, although eventually all are waylaid by interactions with other matter and grind to a halt.

In a paper published in the May 29 issue of Physical Review Letters, Brodsky and Lebed describe two methods by which a type of particle accelerator called an electron-positron accelerator could detect the signature of true muonium's formation and decay. Positrons are the positively charged antiparticles corresponding to electrons.

In the first method, an accelerator's electron and positron beams are arranged slightly askew, to cross each other at a glancing angle. Such a collision would produce a single photon, which would then transform into a single true muonium atom that would be thrown clear of the other particle debris. Because the newly created true muonium atoms would be traveling so fast that the laws of relativity govern, they would decay much more slowly than they would otherwise, making detection easier.

In the second method, the electron and positron beams collide head on. This would produce a true muonium atom and a photon, tangled up in a cloud of particle debris. Yet simply by recoiling against each other, the true muonium and the photon would push one another out of the debris cloud, creating a unique signature not previously searched for.

"It's very likely that people have already created true muonium in this second way," in the course of other experiments, Brodsky said. "They just haven't detected it."

In their paper, Lebed and Brodsky also describe a possible, but more difficult, means by which experimentalists could create another exotic atom, "true tauonium," a bound state of a tau lepton—another elementary particle—and its antiparticle. The tau was first created at SLAC's SPEAR storage ring, a feat for which SLAC physicist Martin Perl received the 1995 Nobel Prize in physics.

Brodsky attributes the pair's successful work to a confluence of events: various unrelated lectures, conversations and ideas over the years, pieces of which came together suddenly during his conversation with Lebed.

"Once you pull all of the ideas together, you say, 'Of course! Why not?'" Brodsky said. "That's the process of science—you try to relate everything new to what you already know, creating logical connections."

Now that those logical connections are firmly in place, Brodsky said he hopes that one of the world's colliders will perform the experiments he and Lebed describe, asking, "Who doesn't want to see a new form of matter that no one's ever seen before?"