Matter-antimatter comparisons reach new heights in groundbreaking study

 

The BASE collaboration at CERN has achieved substantial progress in matter-antimatter comparisons, achieving the world's most exact protons-antiprotons comparison.

The BASE cooperation discusses how it achieved breakthrough success in its matter-antimatter comparisons in an article published in the journal Nature.

This includes determining whether protons and antiprotons - protons' antimatter counterparts – behave in the same way when subjected to gravity.

Measuring Electric charge-to-mass ratios

Over the course of 18 months, researchers at CERN's antimatter factory - a revolutionary facility for antimatter generation and analysis — analysed proton and antiproton observations. The BASE team measured the proton and antiproton's electric charge-to-mass ratios with unprecedented precision, determining that they are equal within an experimental uncertainty of 16 parts per trillion.

"With particles made up of three quarks, known as baryons, and their antiparticles, this result marks the most precise direct test of a basic symmetry between matter and antimatter," said BASE spokesperson Stefan Ulmer.

Matter-antimatter differences: beyond the Standard Model?

Matter and antimatter particles might differ in different ways, such as how they transition into other particles, according to the Standard Model of particle physics, which encapsulates scientists' best hypothesis of particles and their interactions. The majority of their features, such as their masses, should, however, be comparable.

Any tiny discrepancy in the masses of protons and antiprotons, or in the ratios of their electric charge and mass, would violate a basic symmetry of the Standard Model known as CPT symmetry, indicating physics phenomena outside the Standard Model.

The discovery of differences could help scientists comprehend why the Universe is nearly entirely made up of matter, despite the fact that the Standard Model predicts that the Big Bang should have produced equal amounts of matter and antimatter particles.

To explain this apparent cosmic imbalance, the discrepancies between matter and antimatter particles that are consistent with the Standard Model are reduced by orders of magnitude.

Confining matter and antimatter to a Penning trap

Antiprotons and negatively charged hydrogen ions – which are negatively charged proxies for protons – were constrained to a sophisticated particle trap known as a Penning trap in order to make their proton and antiproton observations. In this device, a particle follows a cyclical trajectory with a frequency that scales with the magnetic-field strength of the trap and the particle's charge-to-mass ratio and is near to the cyclotron frequency.

The researchers were able to determine the cyclotron frequencies of antiprotons and negatively charged hydrogen ions under the identical conditions by feeding them one at a time into the trap. This allowed them to compare their charge-to-mass ratios.

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

The scientists discovered that the charge-to-mass ratios of protons and antiprotons are equal to within 16 parts per trillion using these measurements and after taking into account the difference between a proton and a negatively charged hydrogen ion.

"This result is four times more precise than the previous best comparison of these ratios, and the charge-to-mass ratio is now the antiproton's most precisely measured feature," Ulmer stated. "To achieve this precision, we upgraded the experiment significantly and conducted the tests when the antimatter factory was shut down, utilising our antiproton reservoir, which can store antiprotons for years."

When the antimatter factory is not in operation, it is best to conduct cyclotron-frequency measurements since there is no chance that the measurements may be influenced by magnetic field disturbances.

The weak equivalence principle of physics

The BASE researchers used their findings to establish strict restrictions on ideas outside the Standard Model that violate CPT symmetry, in addition to comparing protons and antiprotons with unprecedented precision. They were also able to evaluate the weak equivalence principle, which is a fundamental physics law.

In the absence of friction forces, this principle states that various bodies in the same gravitational field experience the same acceleration. The BASE experiment's proton and antiproton cyclotron-frequency measurements were taken in the gravitational field on the Earth's surface because it is located on the surface. As a result, any difference in protons' and antiprotons' gravitational interactions would result in a difference in proton and antiproton cyclotron frequencies.

The crew found no variation in the Earth's gravitational field as it orbited the Sun and set a maximum value of three parts in 100 for this differential measurement.

"This limit is comparable to the early accuracy targets of experiments aiming to drop antihydrogen in the gravitational field of the Earth," Ulmer stated. "While BASE did not directly dump antimatter into the Earth's gravitational field, our measurement of gravity's influence on a baryonic antimatter particle is conceptually very comparable, revealing no anomalous antimatter-gravity interaction at the attained level of uncertainty."

References:

• Borchert, M.J., Devlin, J.A., Erlewein, S.R. et al. A 16-parts-per-trillion measurement of the antiproton-to-proton charge–mass ratio. Nature 601, 53–57 (2022). https://doi.org/10.1038/s41586-021-04203-w