The magnetic moment of an individual proton is inconceivably small, but can still be quantified. The basis for undertaking this measurement was laid over ten years ago, and physicists of Johannes Gutenberg University Mainz (JGU), the Max Planck Institute for Nuclear Physics, GSI Darmstadt, and the RIKEN research institute in Japan are still performing experiments to measure this force with a single particle to the greatest possible precision. Over the past few years they have refined their experiment even more. By determining the magnetic moment of the proton to ten decimal places, which is the most precise measurement currently available, they set yet another record. The measurements undertaken by Mainz-based physicists as part of the BASE collaboration confirm the Standard Model of particle physics, which describes the smallest particles in our cosmos. The new results for the proton measurement have been published in the journal Science.
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. Magnetic resonance imaging, a technique used in medicine, for example, takes advantage of this property. 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. Beginning in around 2005, Professor Jochen Walz's group at Mainz University has performed experiments using a Penning trap to confine and measure the properties of individual protons to the highest possible accuracy.
Confirmation of the CPT symmetry
The results published in Science have an accuracy of 0.3 parts in a billion, making these new measurements eleven times more precise than the previous measurement made by BASE researchers back in 2014. The g-factor, which characterizes the magnetic moment, was found to be equal to 2.79284734462(82). When this was compared with the value of the g-factor of the antiproton published five weeks ago by the BASE collaboration, no difference between particles and antiparticles was found.
"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," explained Dr. Andreas Mooser from the RIKEN research institute. "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." The so-called CPT symmetry is a fundamental law of physics that predicts that the universe should contain equal amounts of matter and antimatter, which is clearly not the case. "Comparing the current data for protons and antiprotons clearly confirms CPT symmetry," said Mooser.
Measurement comparable with extremely precise magnetic resonance imaging
The Mainz-based physicists achieved the greater precision by making improvements to their technical setup. One innovation was an increase of the homogeneity of the magnetic field even further in the Penning trap, in which the high-precision measurements were carried out. Another was the introduction of a self-shielded coil to reduce external fluctuations. Both measures helped increase the stability of the particle in the trap, allowing the frequencies to be measured with far greater accuracy. "In order to measure the magnetic moment of the proton, we developed one of the most sensitive Penning trap apparatuses ever created," explained Georg Schneider of the Institute of Physics of Mainz University, the first author of the Science publication. "The proton represents a unique challenge as it has such a small magnetic moment. Thus we needed our analysis trap to have an almost inconceivable degree of sensitivity. Basically, you could say that what we have undertaken is extremely precise MRI of an individual proton."
Another improvement was the shortening of the time period until a data point, i.e., a single measurement, was possible. This time per data point was halved from three hours to 90 minutes. "Being able to achieve this level of precision is fantastic, but we are still nowhere near the end of the road," noted Schneider, thus hinting at further advances. In the future, the researchers plan to employ sympathetic laser cooling to reduce the energy of the proton in order to generate greater sensitivity and thus further increase the data collection rates. "The data rate is currently the limiting factor."
The physicists at Mainz University continue to work closely with their colleagues from the BASE collaboration at the CERN research center near Geneva in Switzerland. Positive developments made in Mainz will be shared with CERN and vice versa. The scientists hope to determine the magnetic force of protons and antiprotons even more precisely and either confirm the current model of particle physics or discover a difference, which would open the gate for completely new concepts in physics.