BASE collaboration sets new standards: Matter/antimatter symmetry and antimatter gravity studied at once


This news is based on press releases of the Max Planck Institute for Nuclear Physics, Heidelberg, and the Johannes Gutenberg University Mainz

In the scientific journal Nature, the BASE collaboration at CERN reports on the world's most accurate comparison between protons and antiprotons: The charge-to-mass ratios of antiprotons and protons are identical to eleven digits. This new measurement improves the accuracy of the previous best value by more than a factor of four. The data-set, collected over a period of 1.5 years, also enables a test of the weak equivalence principle, which says that matter and antimatter behave the same under gravity. Researchers from GSI/FAIR are actively involved in the BASE collaboration.

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 and other particle-antiparticle pairs, as observed in many laboratory experiments. In that sense, the existence of our Universe is not self-evident at all. We have 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 the question whether matter and antimatter behave the same under gravity. In their new paper, the BASE scientists compare the similarity of antiproton and proton charge-to-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.

To perform their high-precision studies, the team led by Stefan Ulmer, chief-scientist at RIKEN, Japan, and spokesperson of the BASE collaboration, used a Penning trap, i.e. 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 its mass. Listening to oscillation frequencies of antiprotons and protons in the same trap allows the scientists to compare their masses. “By loading a cylindrical stack of several such Penning traps with antiprotons and negative hydrogen ions, we were able to perform a mass comparison in a measurement time of only four minutes, which means 50 times faster than previous proton/antiproton comparisons by other trap groups,” explains Stefan Ulmer. “Compared to our earlier measurements, we have substantially improved the experimental apparatus. That increases experiment stability and reduces systematic shifts in the measurements.” With this advanced instrument, the BASE team sampled a data set of about 24000 individual frequency comparisons in a time window of 1.5 years. By combining all the measured results, the researchers found that the charge-to-mass ratio of antiprotons and protons is identical, with a precision of 16 parts in a trillion, a number with 11 significant digits. This improves the precision of the best previous measurement, also from BASE, by more than a factor of 4: a significant advance in precision physics.

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 slower. During the long-term measurement of 1.5 years, the Earth, on its elliptic orbit, was exposed to different gravitational potentials of the Sun. With different gravitational behavior of antimatter and matter, the matter and antimatter clocks would experience different frequency shifts along Earth’s planetary trajectory. Analyzing their data, the BASE scientists were not able to find any frequency anomaly. This enabled them to set first direct and largely model-independent limits for anomalous behavior of antimatter in gravitational fields, or, in other words, confirmed the validity of the weak equivalence principle for clocks within the limit of measurement accuracy.

“To measure with even higher precision, we need to move the antiprotons from the accelerator environment of CERN's antimatter factory to dedicated calm laboratory space,” explains Christian Smorra, physicist at the Mainz based PRISMA+ Cluster of Excellence and deputy-spokesperson of BASE, the next steps. “For this purpose, the BASE team is currently constructing the transportable antiproton trap BASE-STEP.” The current plan is to move the antiprotons to a calm laboratory at CERN. If that was successful, the antiprotons can also be distributed to other trap labs. “We will use the transport trap to make even more sensitive tests with antiprotons. In this way, we want to make sure that no new physics with antiprotons will elude us.”

The BASE collaboration consists of scientists from RIKEN Fundamental Symmetries Laboratory, the European Center for Nuclear Research (CERN), the Max Planck Institute for Nuclear Physics in Heidelberg, the Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM), the University of Tokyo, the GSI Helmholtzzentrum in Darmstadt, the Leibniz University Hannover, the Physikalisch-Technische Bundesanstalt (PTB) Braunschweig and ETH Zürich. The research presented now was performed as part of the work of the Max Planck-RIKEN-PTB Center for Time, Constants and Fundamental Symmetries. (MPIK/JGU/BP)

Further information

Scientific publication in Nature

BASE experiment at CERN

Press release of the Max Planck Institut for Nuclear Physics, Heidelberg

Press release of the Johannes Gutenberg University, Mainz