Proton’s magnetic properties pinned down

A lone proton’s magnetism is pretty weak, yet it’s reeling in researchers trying to solve one of the biggest mysteries in physics.

A new measurement quantifies the feeble intrinsic magnetism of the proton and sets the stage for a similar test of the proton’s antimatter counterpart. By comparing this magnetic property of protons with that of antiprotons, researchers hope to gain insight into why the universe is dominated by matter rather than antimatter.

Scientists have a pretty good handle on the proton, but in recent years they have used increasingly sophisticated equipment to probe the particle’s basic properties with unprecedented precision.  One such property is the magnetic moment, which describes how a particle responds to a magnetic field. “Each fundamental particle behaves like a very tiny bar magnet or compass needle,” explains Stefan Ulmer, a particle physicist at RIKEN in Wako, Japan.

Ulmer and his team used a device called a Penning trap to improve on previous magnetic moment measurements for the proton. The trap consists of a small tube emptied of all matter save for a single charged particle — in this case, a proton. Strong electric and magnetic fields lock the proton in place, allowing the researchers to observe the particle responding to subtle changes in the magnetic fields’ strength. The new magnetic moment figure, published in the May 29 Nature, has three times the precision of the previous best measurement, which dates back to 1972.

The proton measurement is impressive, says theoretical physicist Neil Russell of Northern Michigan University in Marquette. He says the work’s most significant contribution is the Penning trap setup: It should also be able to measure the magnetic moment of the antiproton. The 1972 measurement required a low-energy laser fueled by a gas of hydrogen atoms (whose nuclei consist of a single proton each). That technique won’t work for antiprotons because antihydrogen is difficult to produce and harness. Using electric and magnetic fields, however, researchers should be able to isolate and measure the properties of a single antiproton in a Penning trap — and that’s exactly what Ulmer and a team of physicists plan to do within three months using antiproton-producing equipment at CERN, the European particle physics laboratory outside Geneva.

Ulmer says the CERN measurement should improve on the precision of the antiproton magnetic moment by a factor of a thousand. According to the leading theory of particle physics, the antiproton should have the same properties as the proton except for an opposite charge and magnetic moment. The detection of even a minute discrepancy between protons’ and antiprotons’ magnetic moments would represent a subtle difference between matter and antimatter, and perhaps help physicists figure out why the universe is rich in matter but nearly devoid of antimatter. “Finding a difference would be pretty stunning,” Russell says.