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KATRIN
Abstract:
The discovery of neutrino oscillations was a revolutionary milestone in the history of particle physics. Requiring neutrinos to carry a mass, neutrino oscillations are not foreseen in the widely successful standard model of particle physics. Even though neutrino oscillations are not sensitive to the absolute neutrino mass scale, they provide a lower limit of 0.01 eV. Moreover, it is known from β-decay experiments, that the neutrino mass must be tiny. As of today, an upper bound of 0.8 eV could be determined, which is already more than five orders of magnitude smaller than the mass of the next lightest fermion. The lightness of the neutrino brings up the fundamental question of how neutrinos obtain their mass, since the associated mass generation mechanism appears to be different from all other fermions in the standard model. The exact knowledge of the neutrino mass is of uttermost importance to identify the mass generation model that is realized in nature. Moreover, the neutrino mass is a crucial input parameter in cosmology. Neutrinos are the most abundant massive particles in our universe: Each cm3 of the universe is penetrated by 336 neutrinos from the big bang. Despite their small mass, neutrinos have a large influence on the structure formation in the early universe due to their vast abundance. The Karlsruhe Tritium Neutrino (KATRIN) experiment is designed to determine the absolute neutrino-mass scale via the kinematics of molecular tritium β-decay with an unprecedented sensitivity of 0.2 eV at 90 % confidence level after five calendar years. This is achieved in a neutrino-mass model independent way by measuring the integrated β-electron spectrum in the close vicinity of the tritium endpoint at E0 = 18.57 keV. The effective electron antineutrino mass leads to a reduction of the maximal available electron energy and to a slight distortion of the spectral shape, that is most prominent in the endpoint region. To obtain a considerable sensitivity, two cruxes have to be addressed: First, only a small fraction of β-electrons are emitted at the highest energies. Second, the imprint of the neutrino mass in the experimental spectrum is minuscule. The KATRIN experiment tackles these challenges by employing an ultra-stable high-luminosity windowless gaseous tritium source in combination with a spectrometer with an eV-scale energy resolution at a low background level O (0.1 cps). Additionally, a comprehensive understanding of the experimental functionality and of all systematic uncertainties is required. Furthermore, the KATRIN experiment is sensitive to sterile neutrinos with an associated fourth mass eigenstate m4 at the eV mass scale. Sterile neutrinos are a minimal extension of the standard model, forming the right-handed counterpart to the known left-handed neutrino flavors. Due to their handiness, sterile neutrinos do not participate in any fundamental interaction except for gravitation. Thanks to their mixing with the known active neutrinos, their existence can be probed in laboratory experiments. Experimentally, eV-scale sterile neutrinos are well motivated by several anomalies observed in short-baseline neutrino oscillation experiments. The signature of a sterile neutrino in β-decay experiments is a kink-like spectral distortion at electron energies around E0 − m4. In spring and fall 2019, the KATRIN experiment recorded its first two science runs, already collecting more than five million signal electrons in the region of interest. The objective of this thesis is a comprehensive analysis of the first two measurement campaigns in terms of the neutrino mass and the existence of light sterile neutrinos. This thesis is structured as follows: Chapter 1 provides an overview of the key aspects of neutrino physics. The particular focus is put on the neutrino-mass determination with the so-called kinematic approach via single β-decay. Chapter 2 describes the working principle of the KATRIN experiment in detail. In order to perform a high-level data analysis, realistic models of the molecular tritium β-decay and the experimental response are indispensable. Both were implemented in the analysis framework Samak, which was largely developed in this and the author’s previous work. With accumulating statistics, the model required continued revision. In view of the increasing precision, effects become relevant, that only have a tiny impact on the experimental spectrum, such as the energy and angular dependence of the inelastic scattering probabilities. Chapter 3 describes the analysis software and the analysis strategy. Moreover, a large selection of statistical methods for parameter inference, limit calculation and sanity checks were implemented and adapted for the use in KATRIN and subsequently applied to data and simulation. Furthermore, the influence of systematic uncertainties on the model spectrum, for example the uncertainty on the source potential distribution, is evaluated with extensive Monte Carlo simulations and incorporated in the chi-squared analysis with the covariance matrix approach. The individual systematic effects are addressed in detail in chapter 4. The following two chapters (5 and 6) introduce the reader to the first and second measurement campaigns of the KATRIN experiment. During this work, the data taking was supported by various near-time analyses to monitor the global system stability. The characteristic features of the respective measurement campaigns, such as the time evolution of the column density or the observed background rate over-dispersion, are summarized. The blinding protocol, crucial to guarantee a bias-free analysis, is presented in chapter 7. Chapters 8 and 9 are dedicated to the neutrino-mass analyses of the first and second measurement campaigns, respectively. The best-fit results are shown and upper limits on mν are derived. Additionally, numerous studies are carried out to test the robustness of the analysis. The combined analysis of both data sets is presented in chapter 10. The same data sets are further on analyzed with respect to light sterile neutrinos in chapter 11. Exclusion contours for different analysis cases are presented and compared to results from existing experiments. Furthermore, many complementary analyses, for example to investigate the correlation between active and sterile decay branch, are carried out. In chapter 12, the thesis concludes with a summary.
