Proton’s magnetism measured with greatest precision yet


Physicists are raising the bar in the race to find gaps between matter and antimatter.

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Physicists are measuring the fundamental properties of protons and antiprotons to test the principle that antiparticles behave exactly like mirror images of their particle counterparts moving backward in time – and with their electrical charges reversed.
Credit: Alan Stonebraker/APS

In an attempt to solve the mystery of the Universe’s missing antimatter, physicists have made the most accurate measurement yet of the proton’s inherent magnetism.

Post in Nature May 281a group of researchers has mastered a technique to measure the magnetic moment of a proton – the microscopic equivalent of the force of a bar magnet – with an accuracy of 3 parts per billion.

The experiments are part of an effort to understand why the Universe appears to be filled with matter rather than antimatter. Antimatter acts as a mirror image of matter, identical but for the reversal of a few key properties. When the two meet, both are annihilated in a flash of energy. Physicists believe that antimatter and matter would have been produced in equal quantities during the Big Bang; the fact that there is still matter is an enigma.

Any difference between the magnetic moment of the proton and that of the antiproton would reveal an asymmetry which, in the early Universe, could have tipped the balance in favor of matter, says Andreas Mooser, a physicist at Johannes Gutenberg University Mainz. in Germany, a co-author of the study. “The current understanding of physics is that these two values ​​should be equal,” he says.

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Andreas Mooser explains to Elizabeth Gibney how the properties of the proton and antiproton could probe the physics of the early Universe.

The magnetic moment of a proton stems from a fundamental quantum property called spin, which causes the proton to behave like a tiny bar magnet with a north pole and a south pole. When placed in an external magnetic field, the spin of the proton can either align with the field or flip to orient against the field.

The researchers calculated the magnetic moment of the proton by observing a single flip of the proton between these two states. They suspended a proton in a trap and applied a magnetic field that tipped the small bar-shaped magnet. They then pushed the proton into a second trap with a magnetic field gradient and measured its tiny vibrations to determine its spin alignment. By sending the proton back and forth between the traps – performing repeated flips and measurements – the team was able to measure very precisely the frequency at which the magnetic field induced the flip, from which they calculated the magnetic moment. of the proton.

Their figure is 760 times more accurate than the next best direct measurements, made in 2012 by a team led by Gerald Gabrielse, a physicist at Harvard University in Cambridge, Massachusetts.2. It is three times more accurate than the nearest indirect figure, which was derived 42 years ago3.

The experiment is “clearly a breakthrough,” says Ryugo Hayano, a physicist at the University of Tokyo and spokesperson for Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) at CERN, Europe’s near particle physics laboratory. from Geneva, Switzerland. But it’s also just the beginning, he says. “They hope to be able to apply the same method with an antiproton and achieve a similar level of precision,” he says.

Gabrielse’s team has already measured the magnetic moment of the antiproton as part of the ATRAP (Antihydrogen Trap) experiment at CERN and found no difference.4. Mooser and his team can potentially do this with better precision, and their plan is to move their experiment to CERN’s antimatter fabrication facility as part of the Baryon Antibaryon Symmetry Experiment (BASE). When CERN’s facility, known as the Antiproton Decelerator, reopens this summer, ATRAP, BASE and three other groups will use it in a race to spot the tiny gaps between matter and antimatter.

These experiments will also explore other fundamental properties of the gaps between matter and antimatter. These include the electromagnetic emission spectra of antihydrogen and hydrogen and their masses. Hayano says discovering even the tiniest of differences would have “radical” implications for a fundamental theory of physics called CPT symmetry, which predicts that the mass of a particle and its antiparticle should be the same.

Nobody knows how precise experiments have to be before they see nature break this fundamental symmetry – assuming it does. “Recent theories say that this sacred theorem could be violated, but they don’t predict the magnitude of the violation,” Hayano says. “So we’re trying to research many different ways to achieve the highest possible accuracy.”


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Gibney, the magnetism of E. Proton measured with the greatest precision to date.
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