Limit on sterile neutrino contribution from the Mainz Neutrino Mass Experiment

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 light 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

Spectrometer-related background processes and their suppression in the KATRIN experiment

Die direkte Neutrinomassenbestimmung aus dem Tritium-Beta-Zerfall mit dem KATRIN-Experiment erfordert eine niedrige Untergrundzählrate. Die Arbeit beschreibt Entwurf und Optimierung eines komplexen, doppellagigen Drahtelektrodensystems für das KATRIN-Hauptspektrometer zur elektrostatischen Abschirmung der wichtigsten Untergrundkomponente. Ebenso können Fallen für geladene Teilchen zur Untergrundrate beitragen. Eine Methode zur periodischen Fallenentleerung wurde in dieser Arbeit experimentell untersucht. Im Testaufbau wurden mittels einer UV-Leuchtdiode Photoelektronen erzeugt. Dabei wurde gezeigt, dass sich eine schnell pulsbare Photoelektronenquelle mit schmaler Energieverteilung verbunden mit Flugzeitmessungen zur Charakterisierung der Spektrometereigenschaften nutzen lässt. Zusätzlich kann die Winkelverteilung der Elektronen durch geeignete Wahl der elektromagnetischen Felder gesteuert werden. Ein Prototyp einer solchen winkelselektiven Photoelektronenquelle wurde getestet. Zu dieser Arbeit ist ein ergänzender Artikel in der Zeitschrift "New Journal of Physics" erschienen; vgl. http://www.iop.org/EJ/abstract/1367-2630/11/6/063018The direct determination of the neutrino mass scale from tritium beta-decay with the KATRIN experiment requires a very low background level. This work describes the design and optimization of a complex two-layer wire electrode system for the KATRIN main spectrometer with the purpose of suppressing a major background source. Furthermore, charged particles stored in electromagnetic traps in the setup can cause additional background. In a test setup, a method to periodically empty such traps was tested. Photoelectrons created with a UV LED were used to fill the trap. This work demonstrates that such a fast-pulsed photoelectron source with a narrow energy spread can, in combination with time-of-flight measurements, serve as a valuable tool for the characterization of the spectrometer. An appropriate setting of electromagnetic fields allows angular selectivity of the photoelectrons, as shown in tests with a prototype

Probing the Neutrino-Mass Scale with the KATRIN Experiment

The absolute mass scale of neutrinos is an intriguing open question in contemporary physics. The as-yet-unknown mass of the lightest and, at the same time, most abundant massive elementary particle species bears fundamental relevance to theoretical particle physics, astrophysics, and cosmology. The most model-independent experimental approach consists of precision measurements of the kinematics of weak decays, notably tritium β decay. With the KATRIN experiment, this direct neutrino-mass measurement has entered the sub-eV domain, recently pushing the upper limit on the electron-based neutrino mass down to 0.8 eV (90% CL) on the basis of first-year data out of ongoing, multiyear operations. Here, we review the experimental apparatus of KATRIN, the progress of data taking, and initial results. While KATRIN is heading toward the target sensitivity of 0.2 eV, other scientific goals are pursued. We discuss the search for light sterile neutrinos and an outlook on future keV-scale sterile-neutrino searches as well as further physics opportunities beyond the Standard Model

Tritium beta decay with additional emission of new light bosons

We consider tritium beta decay with additional emission of light pseudoscalar or vector bosons coupling to electrons or neutrinos. The electron energy spectrum for all cases is evaluated and shown to be well estimated by approximated analytical expressions. We give the statistical sensitivity of Katrin to the mass and coupling of the new bosons, both in the standard setup of the experiment as well as for future modifications in which the full energy spectrum of tritium decay is accessible