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Physicists Precisely Measure Magnetic Moment of Antiproton

An international team of physicists led by RIKEN researcher Stefan Ulmer has found that the magnetic moment of the antiproton is extremely close to that of the proton, with six-fold higher accuracy than before.

Top: schematic of the Penning trap set-up used in CERN’s BASE experiment; a cloud of antiprotons is stored in the reservoir trap (RT), which supplies single particles to the co-magnetometer trap (CT) and the analysis trap (AT) when required; the CT is used for continuous magnetic field measurements; the AT is the trap with the strong superimposed magnetic bottle, which is used to measure the cyclotron frequency and the Larmor frequency. All traps are equipped with radio-frequency excitation electronics and highly sensitive superconducting detection systems. Bottom: results of the six g-factor measurements carried out during CERN’s 2015/2016 accelerator shutdown between February 20 and March 5, 2016; based on this set of measurements H. Nagahama et al extract gp/2=2.7928465(23), as indicated by the red line; the green lines show the 95% confidence level error, the blue line represents the currently accepted value of the proton g-factor gp/2=2.792847350(9); the error bars of the individual measurements are based on the uncertainties of the individual frequency measurements, which are dominated by the random walk in the magnetron mode. Image credit: H. Nagahama et al, doi: 10.1038/ncomms14084.

Top: schematic of the Penning trap set-up used in CERN’s BASE experiment; a cloud of antiprotons is stored in the reservoir trap (RT), which supplies single particles to the co-magnetometer trap (CT) and the analysis trap (AT) when required; the CT is used for continuous magnetic field measurements; the AT is the trap with the strong superimposed magnetic bottle, which is used to measure the cyclotron frequency and the Larmor frequency. All traps are equipped with radio-frequency excitation electronics and highly sensitive superconducting detection systems. Bottom: results of the six g-factor measurements carried out during CERN’s 2015/2016 accelerator shutdown between February 20 and March 5, 2016; based on this set of measurements H. Nagahama et al extract gp/2=2.7928465(23), as indicated by the red line; the green lines show the 95% confidence level error, the blue line represents the currently accepted value of the proton g-factor gp/2=2.792847350(9); the error bars of the individual measurements are based on the uncertainties of the individual frequency measurements, which are dominated by the random walk in the magnetron mode. Image credit: H. Nagahama et al, doi: 10.1038/ncomms14084.

The idea that something like antimatter must exist came up in the late 1920s. It was only a few years later that positrons, the antiparticles of electrons, were discovered.

While positrons occur naturally on Earth, antiprotons have to be artificially generated.

To perform their experiments, Dr. Ulmer and co-authors took antiprotons generated by CERN’s Antiproton Decelerator and placed them into a powerful magnetic device — called a Penning trap — where they could be stored for periods of more than a year.

When doing the measurements, the researchers took individual antiprotons from the containment trap and moved them into another trap, where they were cooled to practically absolute zero and placed into a powerful and complex magnetic field, allowing the group to measure the magnetic moment.

The moment (g-factor) was determined on the basis of six individual measurements with an uncertainty of just 0.8 parts per million.

The value of 2.7928465(23) is six times more precise than the previous record achieved by another CERN group in 2013. As recently as 2011, the magnetic moment of the antiproton was only known to an accuracy of three decimal places.

The new result is consistent with the g-factor of the proton as measured in Mainz in 2014, namely 2.792847350(9).

“We see a deep contradiction between the Standard Model of particle physics, under which the proton and antiproton are identical mirror images of one another, and the fact that on cosmological scales, there is an enormous gap between the amount of matter and antimatter in the Universe,” said Dr. Ulmer, senior author of the paper reporting the result in the journal Nature Communications this week.

“Our experiment has shown, based on a measurement six times more precise than any done before, that the Standard Model holds up, and that there seems in fact to be no difference in the proton/antiproton magnetic moments at the achieved measurement uncertainty.”

“We did not find any evidence for CPT (charge, parity, time) violation,” he added.

In future experiments Dr. Ulmer and co-authors plan to target the application of an even more sophisticated double Penning trap technique.

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H. Nagahama et al. 2017. Sixfold improved single particle measurement of the magnetic moment of the antiproton. Nat. Commun. 8, 14084; doi: 10.1038/ncomms14084