Matter-antimatter symmetry and antimatter gravity studied at once

BASE collaboration sets new standards / Research group from the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz involved in publication in Nature

10 January 2022


The BASE collaboration at CERN reports in an article in Nature on the comparison of the antiproton-to-proton charge-to-mass ratio with eleven significant digits. This new measurement improves the precision of the previous best value by more than a factor of four, a considerable improvement in precision measurements. The data-set was accumulated over one and a half years of measurement time, allowing for the first differential antiproton/proton test of the Einstein weak equivalence principle of matter and antimatter behaving the same under gravity.

Symmetry and beauty are closely related, not only in music, arts and architecture, but also in the fundamental laws of physics that describe our universe. It is in some sense ironic that our existence seems to be a consequence of a broken symmetry in the best fundamental theory that exists, the Standard Model (SM) of particle physics. One of the cornerstones of the SM is the charge, parity, time (CPT) reversal invariance. Applied to the equations of the SM, the CPT operation translates matter into antimatter. As a consequence of CPT symmetry, matter/antimatter conjugates have the same masses, charges, and magnetic moments, the latter of opposite sign. Another consequence of CPT is that once matter/antimatter conjugates collide, they annihilate to pure energy – as observed in many laboratory experiments. In that sense, the existence of our Universe is not self-evident at all. There is reason to assume that in the Big Bang matter and antimatter were created in equal amounts. Why only matter remained, which makes up the celestial bodies in the universe, has yet to be understood.

Another hot topic in modern physics is whether matter and antimatter behave the same under gravity. In their new Nature article, the BASE scientists compared the similarity of antiproton and proton mass ratios as well as antimatter and matter clocks while the earth was tracing the gravitational potential of the sun, which means that they have simultaneously studied both questions in one measurement.

Do proton and antiproton really have the same mass?

To perform their high precision studies, the team around Dr. Stefan Ulmer, chief scientist at RIKEN, Japan, and spokesperson of the BASE collaboration, used a Penning trap, an electromagnetic container capable of storing and detecting a single quantum of charge. A single particle in such a trap oscillates with a characteristic frequency defined by the mass of the particle. Listening to oscillation frequencies of antiprotons and protons in the same trap allows the scientists to compare their masses. By loading an advanced stack of such Penning traps with antiprotons and negative hydrogen ions, Ulmer's team established a frequency comparison scheme which enables a single mass comparison in a measurement time of only four minutes, 50 times faster than in previous proton/antiproton comparisons by other trap groups. Compared to their earlier measurements, the BASE team has substantially improved the experimental apparatus with several technical upgrades that enhance experiment stability and reduce systematic shifts in their measurements. In this advanced instrument, the BASE team sampled a data set of about 24,000 individual frequency comparisons in a time window of 18 months. Combining all the measured results they were able to compare the antiproton and proton charge-to-mass ratios with a precision of 16 parts in a trillion, a number with eleven significant digits, that improved the precision of the best previous measurement by more than a factor of four.

How does gravity enter the stage?

A particle oscillating in a Penning trap can be considered as a "clock", an antiparticle as an "anti-clock". Clocks at high gravitational potential go faster. During the long-term measurement of 1.5 years the earth on its elliptic orbit was tracing different gravitational potentials of the sun. With different gravitational behavior of antimatter and matter, the matter and antimatter clocks would experience along earths interplanetary trajectory different frequency shifts. Analyzing their data with respect to such beating signatures as a function of time and gravitational potential, the BASE scientists were not able to find any frequency anomaly, which in turn enabled them to set first direct and largely model independent limits on anomalous behavior of antimatter in gravitational fields.

"This interpretation of the data-set constrains the first differential limits on the weak equivalence principle for clocks, at a precision level similar to those anticipated by our colleagues which drop antihydrogen in the gravitational field of the earth", said Dr. Stefan Ulmer. "First experimental results by those experiments will likely become available within the next few antiproton runs. If they would find something while we do not that would lead to another exciting physics puzzle beyond the Standard Model."

To further enhance their capabilities to increase fractional resolution in these type of experiments the BASE team is currently constructing the transportable antiproton trap BASE-STEP. "To measure with even higher precision, the antiprotons need to be moved to dedicated calm laboratory space", said Dr. Christian Smorra, physicist at the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz and BASE deputy spokesperson. "We plan to move the antiprotons out of the accelerator hall into a dedicated precision laboratory at CERN and make even more sensitive tests to make sure that no new physics with antiprotons eludes us."

The BASE collaboration consists of scientists from the RIKEN Fundamental Symmetries Laboratory, the European Center for Nuclear Research (CERN), the Max Planck Institute for Nuclear Physics in Heidelberg, Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM), the University of Tokyo, GSI Darmstadt, Leibniz University Hannover, Physikalisch-Technische Bundesanstalt (PTB) Braunschweig and ETH Zurich. The research was performed as part of the work of the Max Planck-RIKEN-PTB Center for Time, Constants and Fundamental Symmetries, an international group established to develop high-precision measurements to better understand the physics of our Universe.