Magnetic Field Characterisation for Gravitational Free Fall Measurements of Antihydrogen in the ALPHA-g Experiment.
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2024-09-19
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The bound state of an antiproton and positron, antihydrogen, is an ideal test particle for comparisons between matter and antimatter as hydrogen has been studied extensively through history both experimentally and theoretically. The Antihydrogen Laser Physics Apparatus (ALPHA) collaboration has made significant progress on antihydrogen trapping, cooling, and spectroscopy in recent years.In a new apparatus, ALPHA-g, the collaboration aims to probe the effects of gravity on antimatter. This thesis will focus on the first measurement of the free-fall (or gravitational motion) of antimatter and the magnetic field measurements essential to this observation. The ALPHA apparatus, particularly the ALPHA-g experiment, will be discussed in detail. This thesis will outline hardware improvements to the ALPHA-g's Penning trap as well as the microwave injection path. These developments were critical for the operation of ALPHA-g and electron cyclotron resonance (ECR) magnetometry. In addition, a design for a new Penning trap for ALPHA-g is shown that will be key in future measurements. The electron cyclotron resonance (ECR) magnetometry technique used in ALPHA will be introduced. ECR is needed to measure, set, and monitor the magnetic fields in ALPHA-g, ECR is then the essential tool at the center of successful measurements using ALPHA-g. ECR was also used to constrain major systematic uncertainties through a host of applications of the technique in novel ways including fast repeating ECR and multiple simultaneous locations. This work is the first application of this technique in a high magnetic field gradient, which was an important obstacle to overcome in characterising the magnetic fields in ALPHA-g. This breakthrough result of the first ALPHA-g measurement produced a ratio of antihydrogen gravitational acceleration and ``normal gravity" of $a_{\bar{g}}/g =0.75 \pm 0.13(\text{statistical + systematic}) \pm 0.16 (\text{simulation})$. This ratio is consistent (within errors) with no difference between matter and antimatter gravity. This result has allowed ALPHA to rule out extreme difference between matter and antimatter such as no gravitational interaction or repulsive gravity.
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Powell, A. M. W. (2024). Magnetic field characterisation for gravitational free fall measurements of antihydrogen in the ALPHA-g experiment (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.