Session EB: Mini-Symposium: Neutrino Properties and Interactions: Results, Challenges, and Implications I

Neutrino Mass and Sterile Neutrinos

Nearly eighty years after their existence was first proposed, neutrinos remain a powerful laboratory for exploring the Standard Model at the precision and intensity frontiers. Over the next ten years, this field will address a variety of fundamental open issues, many of which will be explored in this mini-symposium. In this partial overview, I will address the search for the neutrino-mass scale, summarizing recent results from KATRIN as well as current challenges and future goals of tritium-based neutrino-mass experiments. I will also survey the search for sterile neutrinos at various scales.   

Updates from the Majorana Demonstrator 0$\nu \beta \beta $ Decay Search Experiment and Its Background Characterization

The Majorana Demonstrator aims to search for neutrinoless double-beta decay (0$\nu \beta \beta )$ of $^{\mathrm{76}}$Ge in large arrays of germanium detectors. It is comprised of two modules of p-type point contact detectors, which are comprised of 15.1 kg of natural and 29.7 kg of $^{\mathrm{76}}$Ge-enriched germanium detectors. With its unprecedented energy resolution of 2.5 keV FWHM and the low background rate of 12 cts/(FWHM t yr) at the $^{\mathrm{76}}$Ge Q-value of 2039 keV, the Demonstrator probes 0$\nu \beta \beta $, searching for new physics beyond the standard model (SM). The Demonstrator has been operating since 2016, at the 4850' level of the Sanford Underground Research Facility. Optimization of background-reducing analysis techniques and the development of a complete background model are expected to improve background rejection, and hence to increase the half-life limit. These background analysis techniques will also inform the design and background expectations in the next-generation LEGEND experiment. In this talk, new results from the improved analysis will be discussed. The model of backgrounds observed by the Majorana Demonstrator and the LEGEND experiment's background expectations from the model will also be discussed.   

Background Rejection through Pulse Shape Discrimination in the Majorana Demonstrator

The \textsc{Majorana Demonstrator} is an experiment searching for neutrinoless double beta decay in $^{76}$Ge. It consists of two modular arrays totaling 44 kg of high purity Ge detectors operating at the 4850’ level of the Sanford Underground Research Facility in South Dakota. The p-type point contact detector technology employed yields sharply-defined pulse characteristics that allow powerful rejection of background event populations. We detail the performance of multisite Compton-scattered gamma background rejection through the AvsE cut parameter, based on the amplitude of the current pulse relative to the total deposited energy. We discuss the systematics associated with this cut and demonstrate its impact on the scientific reach of the experiment.   

Fabrication and Characterization of HPGe Detectors Using Crystals Grown at USD

High purity germanium (HPGe) detectors are widely used in dark matter and neutrino experiments such as CDEX, TEXONO, CoGeNT, COHERENT, GERDA, Majorana, etc. In order to understand and improve the performance of HPGe detectors at various environmental and system configurations in a convenient and economic way, we are in the process of fabricating mini-PPC and planar detectors from HPGe that has been purified with zone refining and grown into HPGe crystals at USD. This way we avoid risking expensive commercial detectors in unconventional operating environments. We take advantage of resources, facilities, and equipment at both USD and Lawrence-Berkeley National Lab. In this presentation, we will describe the process of the fabrication and report our current status and progress.   

Amorphous Germanium Planar Detectors Directly Immersed in LN/LAr for Study of Contact Properties

The stability of an electrical contact when directly immersed in liquid nitrogen (LN) and liquid argon (LAr) is important for the rare event searches including neutrinoless double-beta decay using germanium (Ge) detectors. Utilizing the USD-grown crystals, we fabricated several planar Ge detectors to study electrical contact properties. The fabricated planar Ge detectors were directly immersed in LN and LAr in a test facility at the Max-Planck-Institut fuer physik (MPI) in Munich, Germany. We report that amorphous Ge contacts can survive when the detector is directly immersed. We performed several thermal cycle measurements to study the stability of the contacts. Measurements show that leakage current and energy resolution of the detectors are stable.   

Point Contact Germanium Detector Response to Low-Energy Surface Events

Point contact germanium detectors lead the field in the search for neutrinoless double beta decay, and they have been used to achieve the greatest half-life sensitivity to date. These detectors have excellent energy resolution, low noise, and low-energy threshold. This makes them a great candidate for rare event searches beyond neutrinoless double beta decay as well. One aspect of germanium detectors that is difficult to characterize is their passivated surface. Charge collection near the passivated surface is sensitive to near-surface impurities / defects as well as surface charge buildup. This impacts these detectors' efficiency and resolution, especially in the lower energy realm. At the University of Washington's Center for Experimental Nuclear Physics and Astrophysics we have set out to study this problem. In the investigation reported here, we use a Kr83m source, which has multiple low-energy X-rays below 15 keV, and a number of conversion electrons below approximately 30 keV. We will show first data from this study, and compare it to expectations for different charge collection models. The ultimate goal of this study is to build a model that allows us to characterize the response of a point contact germanium detector to low-energy surface events.   

Level lifetimes determined with the DSAM after fast neutron scattering and relevance to neutrinoless double-beta decay

Neutrinoless double-$\beta$ decay (0$\nu\beta\beta$) has not been observed but is being sought in several large-scale experiments. The nuclear matrix elements for 0$\nu\beta\beta$ cannot be determined experimentally and must be calculated from nuclear structure models. Our recent measurements have focused on providing detailed nuclear structure data to guide these model calculations. At the University of Kentucky Accelerator Laboratory (UKAL), we have performed spectroscopic studies with the (n,n$^\prime\gamma$) reaction on $^{76}$Ge, which is widely regarded as one of the best candidates for the observation of 0$\nu\beta\beta$, and $^{76}$Se, its double-$\beta$ decay daughter. While $^{76}$Ge can be well understood from shell model calculations,$^{76}$Se cannot. To better characterize this transitional region of triaxiality, studies of the lighter stable Ge nuclei, such as $^{74}$Ge, have been initiated. From these measurements, new excited states were identified, level lifetimes were measured with the Doppler-shift attenuation method, multipole mixing ratios were established, and transition probabilities were determined. In the case of $^{74}$Ge, a great deal of information is now available, and shell model calculations explain the low-lying, low-spin structure very well.   

Muon Simulations for LEGEND 1000 Using a GEANT4 Framework

Neutrinoless double beta decay is a proposed rare decay which, if discovered, would confirm the Majorana nature of the neutrino. The LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double-beta decay) collaboration aims to develop a phased, Ge-76 based double-beta decay experimental program with discovery potential at a half-life beyond 10$^{\mathrm{28}}$ years, using existing resources as appropriate to expedite physics results. LEGEND 1000 will be the second phase, and is in the early stages of development. One of the major concerns is the site selection for the experiment. LEGEND 1000 must be built in a deep underground laboratory to escape the bulk of the cosmic rays. A depth requirement analysis is being performed, using the GEANT4 particle simulation toolkit to simulate cosmic ray muons in a few proposed experiment designs. This talk will discuss this ongoing simulation effort.