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The observation of neutrinoless double beta (0 v ββ) decay would establish the Majorana nature of neutrinos and explicitly show that lepton number conservation is violated. In their search for the rare decay in the isotope 76 Ge, the GERDA and MAJORANA DEMONSTRATOR (MJD) experiments have achieved the lowest backgrounds and best energy resolutions in the signal region of interest of any 0 v ββ decay experiment. Building on the successful results of these experiments, as well as contributions from other groups, the Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND) collaboration aims to develop a phased 0 v ββ decay experimental program with discovery potential at a half-life beyond 10 28 years. To achieve this goal, the enriched germanium detector mass has to be increased up to the tonne-scale and the backgrounds have to be reduced further. The first phase of LEGEND, a 200 kg measurement utilizing the existing GERDA infrastructure at LNGS in Italy, is expected to start in 2021.

In this work we present a keV-scale sterile-neutrino search with a low-tritium-activity data set of the KATRIN experiment, acquired in a commissioning run in 2018. KATRIN performs a spectroscopic measurement of the tritium β -decay spectrum with the main goal of directly determining the effective electron anti-neutrino mass. During this commissioning phase a lower tritium activity facilitated the measurement of a wider part of the tritium spectrum and thus the search for sterile neutrinos with a mass of up to 1.6 keV . We do not find a signal and set an exclusion limit on the sterile-to-active mixing amplitude of sin 2 θ < 5 × 10 - 4 ( 95 % C.L.) at a mass of 0.3 keV. This result improves current laboratory-based bounds in the sterile-neutrino mass range between 0.1 and 1.0 keV.

P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector’s response to α particles incident on the sensitive passivated and p+ surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the MajoranaDemonstrator experiment, a search for neutrinoless double-beta decay (0νββ) in 76Ge. α decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of α identification, reliably identifying α background events on the passivated surface of the detector. We demonstrate effective rejection of all surface α events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the 0νββ region of interest window by an order of magnitude in the MajoranaDemonstrator and will be used in the upcoming LEGEND-200 experiment.

The KATRIN experiment aims to determine the effective electron neutrino mass with a sensitivity of \\[{0.2}{\\hbox { eV/c}^{2}}\\] (%90 CL) by precision measurement of the shape of the tritium \\[\\upbeta \\]-spectrum in the endpoint region. The energy analysis of the decay electrons is achieved by a MAC-E filter spectrometer. A common background source in this setup is the decay of short-lived isotopes, such as \\[{}^{\\text {219}}\\text {Rn}\\] and \\[{}^{\\text {220}}\\text {Rn}\\], in the spectrometer volume. Active and passive countermeasures have been implemented and tested at the KATRIN main spectrometer. One of these is the magnetic pulse method, which employs the existing air coil system to reduce the magnetic guiding field in the spectrometer on a short timescale in order to remove low- and high-energy stored electrons. Here we describe the working principle of this method and present results from commissioning measurements at the main spectrometer. Simulations with the particle-tracking software Kassiopeia were carried out to gain a detailed understanding of the electron storage conditions and removal processes.

The neutrino mass experiment KATRIN requires a stability of 3 ppm for the retarding potential at − 18.6 kV of the main spectrometer. To monitor the stability, two custom-made ultra-precise high-voltage dividers were developed and built in cooperation with the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB). Until now, regular absolute calibration of the voltage dividers required bringing the equipment to the specialised metrology laboratory. Here we present a new method based on measuring the energy difference of two 83mKr conversion electron lines with the KATRIN setup, which was demonstrated during KATRIN’s commissioning measurements in July 2017. The measured scale factor M=1972.449(10) of the high-voltage divider K35 is in agreement with the last PTB calibration 4 years ago. This result demonstrates the utility of the calibration method, as well as the long-term stability of the voltage divider.

The projected sensitivity of the effective electron neutrino-mass measurement with the KATRIN experiment is below 0.3 eV (90 % CL) after five years of data acquisition. The sensitivity is affected by the increased rate of the background electrons from KATRIN's main spectrometer. A special shifted-analysing-plane (SAP) configuration was developed to reduce this background by a factor of two. The complex layout of electromagnetic fields in the SAP configuration requires a robust method of estimating these fields. We present in this paper a dedicated calibration measurement of the fields using conversion electrons of gaseous \\(^\\mathrm{83m}\\)Kr, which enables the neutrino-mass measurements in the SAP configuration.

P-type point contact (PPC) germanium detectors are used in rare event and low-background searches, including neutrinoless double beta (0vbb) decay, low-energy nuclear recoils, and coherent elastic neutrino-nucleus scattering. The detectors feature an excellent energy resolution, low detection thresholds down to the sub-keV range, and enhanced background rejection capabilities. However, due to their large passivated surface, separating the signal readout contact from the bias voltage electrode, PPC detectors are susceptible to surface effects such as charge build-up. A profound understanding of their response to surface events is essential. In this work, the response of a PPC detector to alpha and beta particles hitting the passivated surface was investigated in a multi-purpose scanning test stand. It is shown that the passivated surface can accumulate charges resulting in a radial-dependent degradation of the observed event energy. In addition, it is demonstrated that the pulse shapes of surface alpha events show characteristic features which can be used to discriminate against these events.

The fact that neutrinos carry a non-vanishing rest mass is evidence of physics beyond the Standard Model of elementary particles. Their absolute mass bears important relevance from particle physics to cosmology. In this work, we report on the search for the effective electron antineutrino mass with the KATRIN experiment. KATRIN performs precision spectroscopy of the tritium \\(\\beta\\)-decay close to the kinematic endpoint. Based on the first five neutrino-mass measurement campaigns, we derive a best-fit value of \\(m_\\nu^{2} = {-0.14^{+0.13}_{-0.15}}~\\mathrm{eV^2}\\), resulting in an upper limit of \\(m_\\nu < {0.45}~\\mathrm{eV}\\) at 90 % confidence level. With six times the statistics of previous data sets, amounting to 36 million electrons collected in 259 measurement days, a substantial reduction of the background level and improved systematic uncertainties, this result tightens KATRIN's previous bound by a factor of almost two.

Some extensions of the Standard Model of Particle Physics allow for Lorentz invariance and Charge-Parity-Time (CPT)-invariance violations. In the neutrino sector strong constraints have been set by neutrino-oscillation and time-of-flight experiments. However, some Lorentz-invariance-violating parameters are not accessible via these probes. In this work, we focus on the parameters \\((a_{\\text{of}}^{(3)})_{00}\\), \\((a_{\\text{of}}^{(3)})_{10}\\) and \\((a_{\\text{of}}^{(3)})_{11}\\) which would manifest themselves in a non-isotropic beta-decaying source as a sidereal oscillation and an overall shift of the spectral endpoint. Based on the data of the first scientific run of the KATRIN experiment, we set the first limit on \\(\\left|(a_{\\text{of}}^{(3)})_{11}\\right|\\) of \\(< 3.7\\cdot10^{-6}\\) GeV at 90\\% confidence level. Moreover, we derive new constraints on \\((a_{\\text{of}}^{(3)})_{00}\\) and \\((a_{\\text{of}}^{(3)})_{10}\\).

LEGEND, the Large Enriched Germanium Experiment for Neutrinoless \\(\\beta\\beta\\) Decay, is a ton-scale experimental program to search for neutrinoless double beta (\\(0\\nu\\beta\\beta\\)) decay in the isotope \\(^{76}\\)Ge with an unprecedented sensitivity. Building on the success of the low-background \\(^{76}\\)Ge-based GERDA and MAJORANA DEMONSTRATOR experiments, the LEGEND collaboration is targeting a signal discovery sensitivity beyond \\(10^{28}\\,\\)yr on the decay half-life with approximately \\(10\\,\\text{t}\\cdot\\text{yr}\\) of exposure. Signal readout electronics in close proximity to the detectors plays a major role in maximizing the experiment's discovery sensitivity by reducing electronic noise and improving pulse shape analysis capabilities for the rejection of backgrounds. However, the proximity also poses unique challenges for the radiopurity of the electronics. Application-specific integrated circuit (ASIC) technology allows the implementation of the entire charge sensitive amplifier (CSA) into a single low-mass chip while improving the electronic noise and reducing the power consumption. In this work, we investigated the properties and electronic performance of a commercially available ASIC CSA, the XGLab CUBE preamplifier, together with a p-type point contact high-purity germanium detector. We show that low noise levels and excellent energy resolutions can be obtained with this readout. Moreover, we demonstrate the viability of pulse shape discrimination techniques for reducing background events.