The recent analysis of the normalization of reactor antineutrino data, the calibration data of solar neutrino experiments using gallium targets, and the results from the neutrino oscillation experiment MiniBooNE suggest the existence of a fourth ligh
t neutrino mass state with a mass of O(eV), which contributes to the electron neutrino with a sizable mixing angle. Since we know from measurements of the width of the Z0 resonance that there are only three active neutrinos, a fourth neutrino should be sterile (i.e., interact only via gravity). The corresponding fourth neutrino mass state should be visible as an additional kink in beta-decay spectra. In this work the phase II data of the Mainz Neutrino Mass Experiment have been analyzed searching for a possible contribution of a fourth light neutrino mass state. No signature of such a fourth mass state has been found and limits on the mass and the mixing of this fourth mass states are derived.
Recent neutrino mass experiments at Mainz and Troitsk using tritium beta-decay have reached their sensitivity potential, yielding upper limits of about 2 eV/c^2 for the electron antineutrino mass. The KArlsruhe TRItium Neutrino experiment (KATRIN), d
esigned to reach a sensitivity of 0.2 eV/c^2 (90% C.L.), will improve the signal rate by a factor of about 100 with respect to previous experiments while maintaining the same low background level at an enhanced energy resolution of 0.93 eV of the spectrometer which is scaled up by a factor of 10 in linear dimensions. This low background rate can only be achieved by active and passive reduction of the background components induced by the spectrometer itself and in the detector region. Furthermore, sources of systematic errors such as energy losses inside the tritium source or fluctuations of the energy scale of the spectrometer need to be carefully controlled and analysed. An overview of KATRINs method to reduce the background rate and to determine the systematics as well as the sensitivity on the neutrino mass will be presented.
