Вадим Дудченко
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The Standard Model of particle physics is both incredibly successful and glaringly incomplete. Among the questions left open is the striking imbalance of matter and antimatter in the Universe, which inspires experiments to compare the fundamental properties of matter/antimatter conjugates with high precision. The new experiments by the BASE (Baryon Antibaryon Symmetry Experiment) Collaboration at CERN deal with direct investigations of the fundamental properties of protons and antiprotons.

Borchert et al. report on a comparison of the antiproton-to-proton charge-to-mass ratio with a fractional precision of 16 parts per trillion. Image credit: Chukman So.

According to the Standard Model, matter and antimatter particles can differ, for example in the way they transform into other particles, but most of their properties, including their masses, should be identical.

Finding any slight difference between the masses of protons and antiprotons, or between the ratios of their electric charge and mass, would break a fundamental symmetry of the Standard Model, called CPT symmetry, and point to new physics phenomena beyond the Model.

Such a difference could also shed light on why the Universe is made up almost entirely of matter, even though equal amounts of antimatter should have been created in the Big Bang.

The differences between matter and antimatter particles that are consistent with the Standard Model are smaller by orders of magnitude to be able to explain this observed cosmic imbalance.

To make their proton and antiproton measurements, physicists from the BASE Collaboration confined antiprotons and negatively charged hydrogen ions, which are negatively charged proxies for protons, in a state-of-the-art particle trap called a Penning trap.

In this device, a particle follows a cyclical trajectory with a frequency, close to the cyclotron frequency, that scales with the trap’s magnetic-field strength and the particle’s charge-to-mass ratio.

Alternately feeding antiprotons and negatively charged hydrogen ions one at a time into the trap, the researchers measured, under the same conditions, the cyclotron frequencies of these two kinds of particle, allowing their charge-to-mass ratios to be compared.

Performed over four campaigns between December 2017 and May 2019, these measurements resulted in more than 24,000 cyclotron-frequency comparisons, each lasting 260 seconds, between the charge-to-mass ratios of antiprotons and negatively charged hydrogen ions.

From these comparisons, and after accounting for the difference between a proton and a negatively charged hydrogen ion, the BASE team found that the charge-to-mass ratios of protons and antiprotons are equal to within 16 parts per trillion.

“This result is four times more precise than the previous best comparison between these ratios, and the charge-to-mass ratio is now the most precisely measured property of the antiproton,” said Dr. Stefan Ulmer, spokesperson of the BASE Collaboration.

“To reach this precision, we made considerable upgrades to the experiment and carried out the measurements when the antimatter factory was closed down, using our reservoir of antiprotons, which can store antiprotons for years.”

In addition to comparing protons and antiprotons with an unprecedented precision, the scientists used their measurements to place stringent limits on models beyond the Standard Model that violate CPT symmetry, as well as to test a fundamental physics law known as the weak equivalence principle.

According to this principle, different bodies in the same gravitational field undergo the same acceleration in the absence of friction forces.

Because the BASE experiment is placed on the surface of the Earth, its proton and antiproton cyclotron-frequency measurements were made in the gravitational field on the Earth’s surface.

Any difference between the gravitational interaction of protons and antiprotons would result in a difference between the proton and antiproton cyclotron frequencies.

Sampling the varying gravitational field of the Earth as the planet orbits around the Sun, the BASE team found no such difference and set a maximum value on this differential measurement of three parts in 100.

“This limit is comparable to the initial precision goals of experiments that aim to drop antihydrogen in the Earth’s gravitational field,” Dr. Ulmer said.

“BASE did not directly drop antimatter in the Earth’s gravitational field, but our measurement of the influence of gravity on a baryonic antimatter particle is conceptually very similar, indicating no anomalous interaction between antimatter and gravity at the achieved level of uncertainty.”

The team’s results were published in the journal Nature.


M.J. Borchert et al. 2022. A 16-parts-per-trillion measurement of the antiproton-to-proton charge-mass ratio. Nature 601, 53-57; doi: 10.1038/s41586-021-04203-w


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