The Standard Model of particle physics describes all the known fundamental particles and the forces between them, except gravity. A part of this Model – called CPT symmetry – implies that the fundamental properties of particles should be equal and partly opposite to those of their corresponding antiparticles. Any measured difference between the masses, charges, lifetimes, or magnetic moments of matter particles and their antimatter counterparts could contribute to understanding why there is more matter than antimatter in the universe.
The Baryon Antibaryon Symmetry Experiment (BASE) at CERN compares the magnetic moments and charge-to-mass ratios of protons and antiprotons to look for differences in that sector. Using an experimental set-up with four Penning traps – devices that hold particles in place with electromagnetic fields – the BASE collaboration compared proton-antiproton magnetic moments and charge-to-mass ratios with 1.5 parts per billion and 16 parts per trillion precision, respectively.
How does it work?
A direct measurement of the magnetic moment requires the measurements of two frequencies: the Larmor frequency, which characterises the precession of the spin of a particle, and the cyclotron frequency, which describes a charged particle’s trajectory oscillations in a magnetic field.
High measurement precision is achieved thanks to BASE’s multi-Penning trap system:
- The analysis trap identifies the spin state of the particle.
- The precision trap flips the spin of the particle while measuring its cyclotron frequency.
- The cooling trap is capable of cooling the particle rapidly to quantum limited temperatures, allowing for high-fidelity spin state detection.
- The reservoir trap stores typically 50-100 antiprotons (10-25 kilograms) for months on end, allowing the BASE collaboration to continue operating even without beam. In this trap, record antiproton trapping for 600 days has been demonstrated.
The high experiment resolution achieved in BASE is currently limited by magnetic field fluctuations induced by the AD-ELENA decelerators. In order to push proton/antiproton precision studies to ultimate limits, the collaboration has invented a transformative strategy: transporting particles out of the facility to dedicated precision laboratory space, using their recently successfully commissioned transportable antiproton trap BASE-STEP.
Latest results
In 2022, the collaboration has made the most precise comparison yet between the charge-to-mass ratios of protons and antiprotons, and tested whether they behave in the same way under the influence of gravity. The results found these are identical to within an experimental uncertainty of 16 parts per trillion.
In 2024, the experiment has developed a new device for cooling antiprotons more efficiently and considerably increasing the precision of measurements of their fundamental properties; and took a big step towards making antimatter transportable by moving BASE-STEP with unbonded protons across CERN’s main site, thus demonstrating that the same feat could later be possible for antiprotons.[View with recent home.cern news]
In 2025 BASE has demonstrated the first coherent quantum transition spectroscopy with a single antiproton spin and reached a coherence time of 50 seconds. This system, actually the first antimatter quantum bit to be implemented, has the potential to improve the precision of the proton and antiproton magnetic moment measurements at least 100-fold.
