At last week’s Research Board meeting, the Baryon Antibaryon Symmetry Experiment (BASE) was approved for installation at CERN. The collaboration will be setting up shop in the AD Hall this September with its first CERN-based experimental set-up. Using the novel double-Penning trap set-up developed at the University of Mainz, GSI Darmstadt and the Max Plank Institute for Nuclear Physics (Germany), the BASE team will be able to measure the antiproton magnetic moment with hitherto unreachable part-per-billion precision.
“We constructed the first double-Penning trap at our companion facility in Germany, and made the first ever direct observations of single spin flips of a single proton,” says Stefan Ulmer from RIKEN, Japan, the spokesperson of the BASE collaboration. “We also recently demonstrated the first application of the double Penning trap technique with a single proton. This success means we are now ready to use the technique to measure the proton magnetic moment with ultra-high precision and to apply the technique to the antiproton.”
But how does this new trap work? First, let’s look at how the antiproton magnetic moment is derived. A direct measurement of the moment requires two different parameters: the Larmor frequency, which characterizes the precession of the spin of a particle, and the cyclotron frequency, which describes a charged particle's behaviour under a magnetic field.
A strong, homogenous magnet is therefore central to any Penning trap. “Spin flips are observed by coupling the magnetic moment of the particle to the measurable axial frequency, using a magnetic inhomogeneity,” says Ulmer. “As the magnetic moment of the antiproton is so small, a field inhomogeneity of some 300,000 T/m2 is required. However, the intense magnetic field reduces the precision of any frequency measurements.”
The solution? Divide and conquer. BASE’s double Penning trap separates the measurements of the Larmor and the cyclotron frequency from the spin-state analysis. Two traps are used for the measurements: the analysis trap, which will check the spin of the particle, and the precision trap, which will flip the spin of the particle while measuring the cyclotron frequency. In the precision trap, the magnetic field is about 100,000 times more homogeneous than in the analysis one. Thus, this separation dramatically improves the accuracy of the frequency measurements and increases the precision of the magnetic moment.
In addition to these two traps, the experimental set-up will have two further traps. The monitor trap will check for any variance in the magnetic field caused by external sources, allowing the BASE team to make instant adjustments to the core traps while measurements are under way.
The final trap, the reservoir trap, will store antiprotons for months on end, allowing the BASE collaboration to continue operating even without beam. “As BASE is such a sensitive experiment, it may be affected by magnetic field fluctuations from the AD,” says Ulmer. “If that is the case, the reservoir trap will allow us to work when the accelerator is offline.”
This September, the BASE team will begin installing its experiment in the AD Hall. By November, the team plans to be taking new measurements of the proton magnetic moment using an offline source. “It’s an exciting time not only for our collaboration, but also for antimatter physics,” says Ulmer. “As our measurements of antimatter properties grow ever more precise, so too does our understanding of the nature of all matter.”
Layout of the new BASE collaboration set-up to be installed in the AD Hall (Image: BASE)