Neutrino physics harbours a number of unanswered questions, which can be addressed by searches for New Physics in precision experimental data. The theory of General Neutrino Interactions (GNI) offers for this purpose a model-independent approach for a broad search for New Physics. It is a generalisation of the already well-studied neutrino Non-Standard Interactions (NSI), taking into account all mathematically possible types of interactions. These novel interactions are embedded into higher dimensional Standard Model Effective Field Theory (SMEFT) operators, respecting Standard Model (SM) gauge symmetries. Experimentally, these can be investigated through a search for potential shape variations of the $\beta$-decay spectrum.
In this work we present the first proof-of-principle study searching for GNI at the Karlsruhe Tritium Neutrino (KATRIN) experiment. The main purpose of KATRIN is to measure the neutrino mass by precision spectroscopy of the tritium $\beta$-decay with target sensitivity of $0.2\,\mathrm{eV}$. Recently, KATRIN has improved the upper bound on the effective electron-neutrino mass to $0.8\,\mathrm{eV}$ at $90\,\%$ confidence level [1]. This high-precision measurement enables to identify potential spectral modifications such as those caused by GNI by means of energy-dependent contributions to the event rate in KATRIN. The presented studies use simulated data reflecting the measurement conditions of the second KATRIN measurement campaign in 2019, using an energy window down to $40\,\mathrm{eV}$ below the end point of molecular tritium at $18.57\,\mathrm{keV}$. Furthermore, the analysis framework is based on [4], taking into account the existence of a right-handed neutrino.

[1] M. Aker et al., Direct neutrino-mass measurement with sub-electronvolt sensitivity, Nat. Phys. 18 (2022) 160–166 [10.1038/s41567-021-01463-1].
[4] I. Bischer, Effective Neutrino Interactions: Origins and Phenomenology, Doctoral thesis (2021) Heidelberg University [10.11588/heidok.00030163].