PDF, English Download (21MB) | Terms of use

Abstract

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) is a Standard Model process which had long evaded observation at reactor energies due to the tiny recoil energies and challenging background conditions involved. The CONUS+ experiment is a dedicated CE$\nu$NS experiment operating four high-purity germanium detectors inside a massive composite shield located at the commercial nuclear power plant KKL in Leibstadt, Switzerland. The first physics run of the experiment (run 1), which lasted from November 2023 to August 2024 and collected 326.8 kg d of reactor on and 59.7 kg d of reactor off data, yielded the first measurement of CE$\nu$NS for reactor antineutrinos. This thesis first presents the full decomposition and modelling of the background spectra of the CONUS+ detectors in both reactor on and off data taking periods in run 1. The establishment of such a background model is a crucial step towards the full understanding of the measured data and allows the extraction of the CE$\nu$NS signal in a full likelihood analysis, also presented in this work. The findings of this thesis therefore directly enabled and provided the first successful measurement of the interaction at reactor site. \newline Monte Carlo simulations show that the dominant background sources of the experiment are cosmic rays (muons and neutrons), contributing approximately 70 - 80$\%$ of the background rate below 1 keV. At larger energies above the region of interest of the experiment, other sources like the influence of radon in the detector chamber and $^{210}$Pb become more dominant. Reactor-correlated backgrounds are found to be subdominant. The differences in the measured reactor on and off spectra of the detectors were able to be explained by reduced cosmic ray fluxes in the room due to an increased overburden and changes in the radon concentration during the reactor off measurement. The full background model correctly describes the measured detector spectra in both reactor on and off measurements for all energy ranges. The analysis of the CONUS+ run 1 data, which uses these models along with all relevant detector properties and the predicted CE$\nu$NS spectrum, reveals a clear signal at energies below 350 eV, corresponding to (395 $\pm$ 106) neutrino events and excluding the background-only hypothesis at 3.7$\sigma$ confidence level. The observed signal is consistent with the Standard Model prediction within 0.5$\sigma$.