Design and Commissioning of Beamlines for the ALPHA Antihydrogen Experiment


One of the greatest problems facing modern physics is the apparent asymmetry between matter and antimatter. While the standard model of particle physics predicts that equal amounts of matter and antimatter were produced following the Big Bang, astronomical observations have revealed that our universe contains little or no primordial antimatter. Precision measurements of cold, trapped antiparticles can be used to probe fundamental symmetries, and may shed light on why antimatter is so scarce in our universe. The ALPHA experiment at the CERN Antiproton Decelerator studies magnetically trapped antihydrogen atoms, produced by slowly merging cold plasmas of positrons (e +) and antiprotons (¯p). The precision spectroscopy of antihydrogen has already provided unique, high-resolution tests of CPT invariance and theories of new physics beyond the standard model. During 2018, the ALPHA experiment was expanded with the addition of ALPHA-g, a vertical atom trap that is intended to make the first direct measurements of antimatter gravitation.

The efficient transport of ¯p and e + plasmas into the ALPHA-g experiment will be essential to trapping large numbers of antihydrogen atoms. However, the transfer of low energy (. 100 eV) charged particles between separate traps with strong magnetic fields is challenging for a variety of reasons. This thesis describes the design and commissioning of charged particle beamlines for the upgraded ALPHA experiment, and the methods that were used to overcome these challenges.

In this thesis, we first describe the development of a novel magnetic beamline for the ALPHA experiment. Semi-analytical and numerical models are used to characterise the dynamics of ¯p and e + bunches within this beamline. Next, we describe the implementation of the ALPHA-g beamlines at CERN during 2018. We review the specifications of the beamline magnets, and describe the control system used to manage their operation. Finally, a range of diagnostic tools are developed to experimentally measure the properties of actual ¯p and e + bunches. We combine data from a range of sources to evaluate the performance of the beamline, and show that large numbers of positrons and antiprotons can already be delivered to the ALPHA-g experiment. In summary, the work presented in this thesis is critical to a future ±1% measurement of antimatter gravitation, as well as continued precision measurements of antihydrogen atoms.

Mark A. Johnson

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