For the first time, physicists have watched a single proton flip over on its axis. Aside from being a technical triumph, the measurement may eventually help determine why the universe contains more matter than antimatter.

Cosmologists think the Big Bang should have produced the same amount of ordinary matter – the particles that make up stars, planets and people – and antimatter, which is just like matter, only with an opposite charge. But when matter and antimatter meet, they annihilate each other. That there's enough matter left for us to exist is one of modern physics' biggest puzzles.

One possibility is that, opposite charge aside, antimatter isn't always truly identical to matter, and so it doesn't meet the requirements for triggering annihilation. To determine if this is true, physicists need a way to compare matter and antimatter. In a June 24 Physical Review Letters study, physicists take an important step toward comparing protons and antiprotons by measuring a property called the magnetic moment.

"For the proton and the antiproton, magnetic moments have never been compared before," said quantum physicist Stefan Ulmer of the Helmholtz Institute Mainz in Germany, co-author of the new paper. "Our new methods make this comparison possible."

The magnetic moment is a description of how a magnetic field pulls on a particle. It has an intrinsic direction, similar to how a compass needle always points north, but can point up or down depending on what other magnetic forces act on it.

In the past, measuring the strength of the proton's magnetism required clouds of billions or trillions of protons. But the same techniques wouldn't work for antiprotons, because it's hard to keep antimatter around long enough to study it. The current record, announced earlier this month by the Antiproton Decelerator at CERN, is holding 300 anti-hydrogen atoms for 1,000 seconds. That's not nearly enough for magnetic measurements.

Ulmer and colleagues showed that it's possible to measure the magnetic moment of a single proton – and by extension, a single antiproton – by watching it flip back and forth. They accomplished this by confining one proton in a tiny vacuum called a Penning trap, which holds charged particles still using electric and magnetic fields.

A proton usually aligns its magnetic poles with the trap's magnetic field, pointing its northern end upward. But adding an extra magnetic field that spirals in from the side makes the proton do a somersault, pointing its northern end down. How quickly the proton wobbles as it somersaults is proportional to the strength of its magnetic moment.

Now that they know how to measure the magnetic moment of a single proton, the researchers plan to take their device to CERN and try it on antiprotons. Eventually, Ulmer thinks they can make the most precise measurements of the particles' magnetic moments ever.

Similar studies in the 1980s measured the magnetic moment of electrons and their antiparticles, positrons. Those experiments found that the two particles were identical to a precision of two parts in a trillion, or close enough to leave lingering the mystery of why matter and antimatter haven't wiped each other out.

But the proton's magnetic moment is 660 times smaller than the electron's, making Ulmer's experiment a much greater technical challenge. "This requires an extra level of precision, which has to be a factor of 10,000 more precise than everything that has been built so far," Ulmer said. "We have the smallest Penning trap that has ever been built. That's just cool."

If the two particles turn out to have different magnetic moments, "it would invalidate most of modern quantum field theory," said physicist Edmund Myers of Florida State University, who was not involved in the new study. "Theorists would say it's extremely unlike that such a discrepancy would be seen. But that makes the experimentalists all the more excited to look for one."

Images: 1) A drawing of the magnetic field lines inside a Penning trap. 2) The Penning trap itself. Courtesy Stefan Ulmer

Citation: "Observations of Spin Flips with a Single Trapped Proton." S. Ulmer, C.C. Rodegheri, K. Blaum, H. Kracke, A. Mooser, W. Quint and J. Walz. Physical Review Letters, Vol. 106, No. 253001. June 20, 2011. DOI: 10.1103/PhysRevLett.106.253001.

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