CERN’s BASE Experiment Reports 350-Fold Improved Measurement of Antiproton’s Magnetic Moment

In a paper published this week in the journal Nature, the BASE collaboration at CERN reports the most precise measurement ever made of the magnetic moment of the antiproton. The new measurement outperforms the previous best measurement by a factor of 350 in experimental precision.

Artist’s impression of a cloud of trapped antihydrogen -- atoms comprised of antiprotons and positrons. Image credit: Chukman So.

Artist’s impression of a cloud of trapped antihydrogen — atoms comprised of antiprotons and positrons. Image credit: Chukman So.

The magnetic moment of the antiproton is found to be 2.7928473441(42), to be compared to the figure of 2.792847350(9) that the same collaboration of researchers found for the proton in 2014.

The results are consistent with the magnetic moments of the proton and antiproton being equal, with the experimental uncertainty of the new antiproton measurement now significantly smaller than that for protons.

“Our updated value is consistent with the magnetic moment of the proton, and thus supports the combined charge, parity, and time-reversal (CPT) invariance, an important symmetry of the Standard Model of particle physics,” BASE collaboration members said.

“Remarkably, this is the first time physicists carried out a more precise measurement on antiprotons than on protons.”

“Together with the exciting new antihydrogen results, this milestone achievement is a demonstration of the immense progress made at CERN’s antiproton decelerator facility.”

To perform the measurement, the physicists used a novel, elegant, two particle measurement method.

“This extraordinary improvement in experimental accuracy was made possible by the invention of a novel two-particle multi-Penning-trap measurement method,” they said.

“The system involves the simultaneous trapping and measurement within an even magnetic field of two separate antiprotons — one measured at a relatively high temperature of about 350 degrees Kelvin (a temperature equivalent to hot water) — and the other at just 0.15 K (extremely close to absolute zero).”

“The first antiproton is used to calibrate the magnetic field, by measuring a property called the ‘cyclotron frequency,’ while the other is used to measure a quality known as the Larmor frequency, which looks at the precession of the particle’s spin, allowing precise measurements of the magnetic moment.”

“This result is the culmination of many years of continuous research and development, and the successful completion of one of the most difficult measurements ever performed in a Penning trap instrument,” said Dr. Stefan Ulmer, spokesperson of the BASE collaboration.

“It is probably the first time that physicists get a more precise measurement for antimatter than for matter, which demonstrates the extraordinary progress accomplished at CERN’s Antiproton Decelerator,” added lead author Dr. Christian Smorra, of RIKEN laboratory in Japan.

“All of our observations find a complete symmetry between matter and antimatter, which is why the Universe should not actually exist,” he added.

“An asymmetry must exist here somewhere but we simply do not understand where the difference is. What is the source of the symmetry break?”

“We now want to use even higher precision measurements of the proton and antiproton properties to find an answer to this question,” the researchers said.

“We plan to develop further innovative methods over the next few years and improve on the current results.”


C. Smorra et al. 2017. A parts-per-billion measurement of the antiproton magnetic moment. Nature 550: 371-374; doi: 10.1038/nature24048

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