Particle physics experiments

Antimatter reacts to gravity the same way ordinary matter does, according to new experiments

The Standard Model of particle physics is both incredibly successful and patently incomplete. Among the questions left open is the striking imbalance between matter and antimatter in the Universe, which inspires experiments to compare the fundamental properties of matter / antimatter conjugates with great precision. The new experiments of the BASE collaboration (Baryon Antibaryon Symmetry Experiment) at CERN focus on direct studies of the fundamental properties of protons and antiprotons.

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

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

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

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

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

To perform their proton and antiproton measurements, physicists in the BASE Collaboration confined negatively charged antiprotons and hydrogen ions, which are negatively charged proxies of protons, in an advanced particle trap called the Penning trap.

In this device, a particle follows a cyclic trajectory with a frequency, close to the cyclotron frequency, which changes with the strength of the magnetic field of the trap and the charge / mass ratio of the particle.

By feeding antiprotons and negatively charged hydrogen ions in turn into the trap, the researchers measured, under the same conditions, the cyclotron frequencies of these two types of particles, making it possible to compare their charge / mass ratios.

Carried out over four campaigns between December 2017 and May 2019, these measurements gave rise to more than 24,000 cyclotron frequency comparisons, each lasting 260 seconds, between the charge / mass ratios of the antiprotons and of the negatively charged hydrogen ions.

From these comparisons, and after taking into account 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 the nearest 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 for the collaboration. BASED.

“To achieve this accuracy, we dramatically improved the experiment and performed the measurements during the shutdown of the antimatter factory, using our antiproton reservoir, which can store antiprotons for years.”

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

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

Since the BASE experiment was placed on the surface of the Earth, its proton and antiproton cyclotron frequency measurements were carried out in the gravitational field at the surface of the Earth.

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

By sampling Earth’s varying gravitational field as the planet orbits the Sun, the BASE team found no such difference and set a maximum value on this differential measurement of three out of 100 parts.

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

“BASE did not directly drop antimatter into Earth’s gravitational field, but our measure of the influence of gravity on a baryon antimatter particle is conceptually very similar, indicating no abnormal interactions between l ‘antimatter and gravity to the level of uncertainty reached. “

The team’s results have been published in the journal Nature.


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