Measuring the magnetic moment of a single proton

Wired : 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. When matter and antimatter meet, they annihilate each other. One possible explanation for the predominance of matter, opposite charge aside, is that antimatter isn’t always truly identical to matter. To determine if this is true, physicists need a way to compare matter and antimatter. In a study published 24 June in Physical Review Letters, physicists take an important step toward comparing protons and antiprotons by measuring the magnetic moment.The magnetic moment is a description of how a magnetic field pulls on a particle. Stefan Ulmer of the Helmholtz Institute Mainz in Germany 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. 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.