Matter-antimatter comparisons reach new heights in groundbreaking study

CERN’s BASE collaboration has made significant steps in matter-antimatter comparisons, making the world’s most precise comparison of protons and antiprotons.

In a paper published in the journal Nature, the BASE collaboration reveals how it achieved groundbreaking success in its matter-antimatter comparisons.This included evaluating whether protons and antiprotons – the antimatter counterparts of protons – behave similarly under the influence of gravity.

Measuring electric charge-to-mass ratios

Researchers at CERN’s antimatter factory – a novel facility for antimatter production and analyses – have spent over 18 months analysing proton and antiproton measurements. The BASE team assessed the electric charge-to-mass ratios of the proton and antiproton with record breaking precision, discovering they are identical, within an experimental uncertainty of 16 parts per trillion.

“This result represents the most precise direct test of a fundamental symmetry between matter and antimatter, performed with particles made of three quarks, known as baryons, and their antiparticles,” explained BASE spokesperson Stefan Ulmer.

Matter-antimatter differences: beyond the Standard Model?

According to the Standard Model of particle physics – which epitomises scientists’ best theory of particles and their interactions – matter and antimatter particles can differ in various ways, such as how they transform into other particles. However, the majority of their properties, including their masses, should be identical.

The discovery of any minor 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, known as CPT symmetry, and indicate physics phenomena beyond the Standard Model.

The revelation of differences may help scientists understand why the Universe is comprised almost entirely of matter, even though the Standard Model suggests equal amounts of matter and antimatter particles should have generated 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 clarify this observed cosmic imbalance.

Confining matter and antimatter to a Penning trap

In order to make their proton and antiproton measurements, the team limited antiprotons and negatively charged hydrogen ions – which are negatively charged proxies for protons – to an advanced particle trap known as a Penning trap. In this device, a particle undertakes 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.

By consecutively feeding antiprotons and negatively charged hydrogen ions one at a time into the trap, the group was able to measure – under the same conditions – the cyclotron frequencies of these two kinds of particle, enabling comparison of their charge-to-mass ratios.

The measurements – which were attained over four campaigns between December 2017 and May 2019 – led to more than 24,000 cyclotron-frequency comparisons, each lasting 260 seconds, between the charge-to-mass ratios of antiprotons, and negatively charged hydrogen ions.

Using these comparisons – and after considering the difference between a proton and a negatively charged hydrogen ion – the team discovered 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.” added Ulmer. “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.”

Carrying out cyclotron-frequency measurements when the antimatter factory is not in operation is optimal, as there is no possibility for the measurements to be impacted by disturbances to the experiment’s magnetic field.

The weak equivalence principle of physics

On top of comparing protons and antiprotons with an unparalleled precision, the BASE researchers utilised their measurements to place stringent limits on models beyond the Standard Model that violate CPT symmetry. As well as this, they were able to assess a fundamental physics law called the weak equivalence principle.

This principle entails that different bodies in the same gravitational field endure 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. Thus, any difference between the gravitational interaction of protons and antiprotons would lead to a difference between the proton and antiproton cyclotron frequencies.

Testing the differing gravitational field of the Earth as it orbits the Sun, the team discovered 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,” concluded Ulmer. “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.”

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