Studies of muonium emission into vacuum and diffusion of muonic hydrogen in the μp hyperfine splitting experiment at PSI
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2023
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Doctoral Thesis
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Abstract
Muonium (M = µ⁺ + e⁻) and muonic hydrogen (µp p⁺ + µ⁻) are exotic versions of the hydrogen atom, in which the proton or electron are replaced with an antimuon or a muon respectively. In muonium, the antimuon serves as a leptonic nucleus without any measurable size or electro-magnetic sub-structure, which makes it an excellent atomic system for precision tests of quantum electrodynamics and indirect searches for new physics. Muonic hydrogen, on the other hand, is eminently suited for precision studies of the proton’s internal structure, to which its atomic states are more sensitive due to the larger mass of the muon compared to the electron. Currently, a number of fundamental physics experiments with M and µp atoms are in preparation at the Paul Scherrer Institute (PSI). Although these experiments may have implications for physics at high energy scales, their measurement principles all involve low-energy processes, such as exotic atoms diffusing through the target after formation or travelling through vacuum at thermal energies. This thesis is mainly concerned with such low-energy processes and presents studies of the emission of M atoms from target samples at room temperature, as well as simulations of the diffusion of µp atoms through hydrogen gas.
The LEMING Collaboration is currently preparing an experiment at PSI with the aim of measuring the gravitational interaction of muonium atoms with the Earth. While much effort in recent years has been dedicated to the development of a cryogenic muonium source using superfluid helium, additional measurements of muonium emission into vacuum from conventional target samples were carried out at room temperature during test beams in 2018 and 2020. The room-temperature measurement in 2018 was carried out before the start of this PhD thesis. Detection of M decays in vacuum was performed using small scintillator bars to detect decay positrons in combination with a system to accelerate and detect the atomic electrons in coincidence. In 2020, two different roomtemperature setups were used, which had both been developed within this PhD project. One of these setups featured an optimized arrangement of scintillator bars to detect M decays with increased efficiency and without the need for an atomic electron detection system. The other setup contained two Micromegas tracking detectors used in a telescope configuration to obtain an image of the cloud of M decays in vacuum. In this thesis, the three room-temperature studies are discussed and analyses of the measured data are presented. Comparing measured data with simulations, emission characteristics of the tested laser-ablated aerogel samples and zeolite samples are examined and muon-tovacuum-muonium conversion efficiencies are extracted. With that, this thesis contributes to the research field investigating muonium emission from novel target materials.
For the development of a cryogenic muonium source with superfluid helium, elastic scattering of M atoms with ⁴He atoms in the gas phase must be taken into account. A measurement of the scattering strength was pursued during the test beam in 2018 by observing M emission into low-density helium gas. Within this thesis, calculations of the elastic cross sections for M–⁴He scattering are carried out assuming an empirical interaction potential. The cross sections were used for the implementation of M–⁴He scattering simulations in G4BEAMLINE. Within uncertainties, good agreement between the simulations and measured data provide validation of the calculated cross sections and the underlying scattering potential.
Another experiment which is currently being prepared at PSI is a measurement of the ground-state hyperfine splitting (HFS) in muonic hydrogen. This is part of a comprehensive effort by the CREMA Collaboration to further the understanding of the proton structure with laser spectroscopy experiments using muonic atoms. In this particular experiment, negative muons are stopped in a cryogenic hydrogen gas target and form µp atoms, which then diffuse through the target. µp atoms reaching one of the gold-coated target walls lead to muonic gold x rays which can be measured outside the target cell. Exciting the diffusing µp atoms with a laser on the resonance frequency of the HFS transition increases their probability of reaching one of the walls. A peak in the x-ray yield consequently occurs at the resonance frequency and thus allows for precise determination of the desired HFS transition frequency. In the scope of this thesis, simulations of the µp diffusion process through the hydrogen gas were implemented and carried out in G4BEAMLINE. These simulations use differential cross sections for the scattering of µp atoms with hydrogen molecules which were calculated by a collaborator. In combination with inputs of theoretical and experimental origin, the diffusion simulations are used to predict the event rates of signal and background to be expected in the measurement. Based on these rates, the measurement time needed to find the resonance and the precision which can be reached in the measurement are estimated. With that, the simulations provide important input for the planning of the measurement campaign and for the further development of the experimental system.
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03864 - Kirch, Klaus / Kirch, Klaus