In Physical Review Letters, the Antihydrogen TRAP (ATRAP) experiment at CERN's Antiproton Decelerator (AD) reports a new measurement of the antiproton magnetic moment made with an unprecedented uncertainty of 4.4 parts per million (ppm). This result is 680 times more precise than previous measurements. The unusual increase in precision is due to the experiment’s ability to trap individual protons and antiprotons, and to use a huge magnetic gradient to gain sensitivity to the tiny magnetic moment. ATRAP’s new result is partly an attempt to understand the matter-antimatter imbalance of the universe, one of the great mysteries of modern physics.

Using a device called a Penning trap, a sort of electromagnetic cage, the antiproton is suspended at the centre of an iron ring electrode sandwiched between copper electrodes. Thermal contact with liquid helium keeps the electrodes at 4.2 K, providing a nearly perfect vacuum that eliminates the stray matter atoms that could otherwise annihilate the antiproton. Static and oscillating voltages applied to the electrodes allow the antiproton to be manipulated and its properties to be measured.

The ATRAP team found that the magnetic moments of the antiproton and proton are "exactly opposite": equal in strength but opposite in direction with respect to the particle spins, consistent with the prediction of the Standard Model and its CPT theorem to 5 parts per million. However, the potential for much greater measurement precision puts ATRAP in position to eventually test the Standard Model prediction much more stringently.

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