New Physics with Neutrinos
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Abstract
Due to open problems in theory and unresolved anomalies in various oscillation experiments, neutrino physics appears to be an excellent starting point in the quest for physics beyond the Standard Model. The 3+1 framework featuring an additional, sterile neutrino mixing with the known neutrinos, is a minimalist extension of the Standard Model which can account for oscillation phenomenology beyond the standard three flavor paradigm. However, two orthogonal arguments militate against this model.
First, the increasing amount of ambiguous experimental results makes a consistent interpretation of the data in the 3+1 framework appear unlikely. In this regard, global fits provide a useful tool to investigate this objection. This thesis reports on the results of an up-to-date global fit to all relevant, available datasets, including the data corresponding to the reactor antineutrino anomaly, the gallium anomaly and the short baseline anomaly. The reactor data as a special case can plausibly be explained by the hypothesis of a misprediction of the reactor antineutrino flux. Therefore, this hypothesis is tested against the 3+1 framework. Both hypotheses are found to be similarly likely, with a slight preference for the 3+1 framework, mainly driven by the data from DANSS and NEOS, which measure antineutrino spectra. However, a combination of both hypotheses fits the data best, with the hypothesis of a mere misprediction of the reactor flux rejected at $2.9 sigma$. Despite mild tensions, adding the remaining datasets in the $ parenbar nu_e$ disappearance channel increases the evidence for the 3+1 framework to $3.2 sigma$. This result is independent from any prediction on the reactor flux. Similarly, the data taken in the $parenbar nu_e$ appearance channel favor the 3+1 framework with a significance of up to $6.5 sigma$, depending on the LSND datasets included, although the goodness of fit is poor. These results are in strong contrast to the lack of evidence for the 3+1 framework in the $ parenbar nu_mu$ disappearance channel. Especially driven by the new results from IceCube and MINOS, this lack of evidence allows for rigorous constraints on the parameter space in the $ parenbar nu_mu$ disappearance channel. The combined fit to the global data proves that the different subsets are incompatible with each other at the $4.7sigma$ level. This tension is in particular driven by LSND and robust with regard to the choice of the fitted dataset and the underlying reactor flux model.
The second objection to the 3+1 framework are the strong constraints on the effective number $N_text{eff}$ and the sum of masses $ sum m_nu$ of neutrino-like particles implied by different cosmological probes. These constraints are in conflict with the presence of an additional, sterile neutrino species in the early universe. However, if their production was prevented throughout the evolution of the universe, the constraints from cosmology would not apply for sterile neutrinos. The generation of sterile neutrinos can be suppressed by a new type of interaction, termed secret interaction''. Still, as this thesis confirms, the secret interaction model is disfavored in all of the viable parameter space. Therefore, inverse symmetry breaking, additional sterile neutrinos, additional free-streaming particles or neutrino decay are proposed as potential extensions of the secret interaction model.