Physicists Precisely Measure Proton’s Magnetic Moment

In a paper published in the journal Science, a team of physicists from Japan and Germany reports the most precise measurement ever made of the magnetic moment of the proton, allowing a fundamental comparison between matter and antimatter. The new result improves by a factor 11 the precision of the previous measurement, and is consistent with the currently accepted value.

Proton is a subatomic particle found in the nucleus of every atom. This artist’s impression shows a proton and a neutron. Image credit: Joanna Griffin / Jefferson Lab / Penn State.

Protons are positively-charged particles in atomic nuclei. In addition to an electric charge they also have an intrinsic angular momentum, the spin, giving them a magnetic moment.

Although this fundamental property of the proton has no direct implication for current technology, it is instead of far greater significance for understanding atomic structures and for precisely testing fundamental symmetries in the Universe, in particular the imbalance of matter and antimatter.

“Knowing the properties of the proton such as its mass, lifetime, charge, radius, and its magnetic moment as precisely as possible is extremely important for physics,” said Dr. Andreas Mooser, a postdoctoral researcher at RIKEN in Japan.

“High-precision measurements of all these properties can provide us with the foundations to be able to more precisely investigate fundamental symmetries such as charge, parity, and time reversal symmetry.”

Dr. Mooser and co-authors used an optimized double Penning trap to determine the magnetic moment of a single proton to a precision of 0.3 parts per billion.

The updated value 2.79284734462(82) is consistent with the magnetic moment of the antiproton 2.7928473441(42), and thus supports the combined charge, parity, and time-reversal (CPT) invariance, an important symmetry of the Standard Model of particle physics.

“In order to measure the magnetic moment of the proton, we developed one of the most sensitive Penning trap apparatuses ever created,” said Dr. Georg Schneider, fro the Institute of Physics at Mainz University, Germany.

“First, we had to isolate a single proton in the trap. We did this by detecting the thermal signal of the ions stuck in the trap, and then using an electric field to eliminate them until we were left with just one,” the physicists said.

“The key to the tremendous precision, however, was a combination of extremely difficult engineering coupled with the ability to shuttle the proton between two different traps.”

“Our method for directly measuring the magnetic moment of a particle is based on the fact that a proton in a Penning trap aligns its spin with the trap’s magnetic field.”

“The basic method is to use the detector to measure two frequencies: the Larmor (spin-precession) frequency and the cyclotron frequency of the proton in a magnetic field. These can be used to find the magnetic moment.”

“The cyclotron frequency of the proton can be measured using what is called the Brown-Gabrielse invariance theorem, while the Larmor frequency can be measured by driving spin flips — using a radio frequency signal that heats the particle — and measuring the probability of a spin flip as a function of the drive frequency.”

“The already high precision of these measurements can be boosted further, however, by using the double-trap method, where the cyclotron frequency is measured and spin transitions are induced in a first trap.”

“The proton is then carefully shuttled to a second trap, where the spin state is detected using a large magnetic inhomogeneity — a magnetic bottle.”

“The spatial separation of high-precision frequency measurement and spin state detection makes the extremely precise measurements possible.”

“To move forward in particle physics, we require either high-energy facilities or super precise measurements,” Dr. Schneider said.

“With our work we are taking the second route, and we hope in the future to do similar experiments with antiprotons using the same technique. This will allow us to get a better understanding of, for example, atomic structure.”

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Georg Schneider et al. 2017. Double-trap measurement of the proton magnetic moment at 0.3 parts per billion precision. Science 358 (6366): 1081-1084; doi: 10.1126/science.aan0207