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BASE - Precision Experiments with Antimatter

Our group is leading the international BASE collaboration at CERN, here we use advanced Penning trap systems to compare the fundamental properties of protons and antiprotons with ultra-high precision. These experiments are inspired by the striking imbalance of matter over antimatter in our Universe, whose origin has yet to be discovered. Combining the best fundamental physics theories, the Standard Model (SM) of particle physics and the Lambda-CDM model, we would expect that the big bang has created equal amounts of matter and antimatter, which would have annihilated afterwards to form a purely radiative Universe. On the other hand, we observe a matter dominated Universe with a baryon to photon density that is by more than eight orders of magnitude smaller than theory predicts.

In our experiments we contribute to understanding this puzzle and compare the fundamental properties of protons and antiprotons in advanced multi-Penning-trap systems. We have invented a Reservoir Penning trap technique, that enables us to store antiprotons for years in a background-vacuum better than 10-18 mbar. This method allows us to perform precision experiments with antiprotons throughout the entire year, independent of accelerator cycles. In addition, we have developed advanced magnetic shielding methods and ultra-sensitive detection techniques to non-destructively observe single antiprotons on macroscopic time scales, and to determine their properties with highest possible precision. Most importantly, we have developed Penning traps with ultra-strong magnetic gradients and outstanding-noise performance, that allow us to perform non-destructive nuclear magnetic resonance spectroscopy with single antiparticle spins, a key ingredient to measure magnetic moments with outstanding accuracy.


Using these techniques, we have applied a novel two particle/three trap method and measured the antiproton magnetic moment with a fractional accuracy of nine significant digits. Together with our proton magnetic moment measurements, this improves the previous best tests of matter/antimatter asymmetry in that sector by more than a factor of 3000. In addition, we have determined the proton/antiproton charge to mass ratios with a fractional accuracy of 16 parts in a trillion, which constitutes the current best test of the fundamental charge, parity, time (CPT) reversal invariance in the baryon sector. This measurement also constitutes the first differential test of the weak equivalence principle for baryonic antimatter. In case the weak equivalence principle would be violated for antimatter, compared to proton clocks, antiproton clocks would experience a different gravitational redshift. We have compared proton/antiproton cyclotron clocks for more than a year, while the earth is moving on its elliptical orbit around the sun, changing the gravitational potential in the laboratory. This measurement sets stringent limits on WEP-violation for antimatter.

Supported by future techniques, partly developed at the University of Hannover and at the University of Mainz, we are currently implementing methods to sympathetically cool protons and antiprotons, by coupling them to laser cooled Be ions. On the long term, this may allow the implementation of quantum logic readout of proton and antiproton spins, increasing the sampling rate and reducing systematics in future measurement campaigns.

BASE-STEP: Transportable Antiproton Traps (ERC Project, C. Smorra)

The resolution of precision experiments on antimatter systems is ultimately limited by technical (magnetic) noise in the environment of CERN’s accelerator structures. Thus, we are developing for future generation experiments the transportable antiproton trap BASE-STEP (Symmetry Tests in Experiments with Portable antiprotons), with the goal to move antiparticles to calm laboratory space. The ultimate goal of this initiative, led by Christian Smorra (ERC Strarting Grant Project Leader), it to distribute antiprotons over several laboratories in Europe, to form a network of precision spectroscopy experiments that perform synchronized measurements.

Dark Matter Haloscope (BASE-CDM)

Astrophysical observations imply that the celestial bodies and stars (baryonic matter) we are able to detect with our electromagnetic senses make only about 5% of what actually exists, 68% consist of a mysterious substance called “dark energy” and the remaining 27% are made of “dark matter”. Although our understanding of the Universe provides clear evidence for the existence of dark matter, its microscopic properties have never been observed. Axions and axion-like particles, motivated by several beyond standard model theories, are candidates for dark matter. These light particles convert in strong magnetic fields to standard model photons which can potentially be detected with the ultra-sensitive superconducting particle detectors used in Penning trap experiments. Using our detector technology, we are currently setting up the axion haloscope BASE CDM, to search for axion like particles in the mass range between nano-eV and micro-eV.

Collaboration Projects

We are member of the Max Planck, RIKEN, PTB Center for Time, Constants and Fundamental Symmetries, and collaborate with other groups within the center and beyond.

  • Direct measurement of the magnetic moment of 3He2+ (Mooser / Blaum - MPIK)
  • High precision mass spectrometry on highly charged ions at PENTATRAP (Blaum – MPIK)
  • Through RIKEN we are involved in the antihydrogen research of the ASACUSA collaboration at CERN, with the goal to measure the ground state hyperfine splitting of antihydrogen with parts per million precision.
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