Session LM: Mini-Symposium: Neutrinos II

The nEXO Outer Detector and Muon Veto

The nEXO experiment is a next-generation neutrinoless double-beta decay search with the isotope $^{\mathrm{136}}$Xe and a half-life sensitivity goal of 10$^{\mathrm{28}}$ years. The nEXO experiment plans to take full advantage of the self-shielding effects of the liquid xenon and exploit as large a fiducial mass as possible; therefore minimizing external contributions to the background radiation entering the nEXO time projection chamber (TPC) is required. In order to accomplish this task, an outer detector, in which the nEXO TPC and cryostat are fully submerged, consisting of a tank filled with ultra-pure deionized water and instrumented with 8-inch PMTs will provide the outer shielding and act as a muon veto for nEXO. The initial design along with the passive and active shielding capability for external backgrounds will be presented.   

An Improved Background Model for the MAJORANA DEMONSTRATOR

The MAJORANA collaboration is searching for neutrinoless double-beta decay in Ge-76 using modular arrays of enriched, high-purity Ge detectors. The detectors are distributed between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. A low background index has been achieved at the 0$\nu\beta\beta$ Q-value, and low backgrounds in the low-energy region near threshold have enabled searches for other beyond the standard model processes. Background model fits using two different statistical approaches aim to determine the sources of observed backgrounds and to explain deviations from assay-based projections. Probability density functions of sources distant from the detector array have been introduced to the model through high-statistic simulations. These pdfs allow the model to address low-efficiency shine paths that were difficult to address in the past. The improved background model presented here will help inform design choices and background considerations for the next-generation LEGEND experiment.   

New Results in the search for $0\nu\beta\beta$ decay in $^{100}$Mo from CUPID-Mo

CUPID-Mo is a demonstrator experiment for CUPID (CUORE Upgrade with Particle ID), the planned next-generation upgrade of the first ton scale cryogenic bolometric $0\nu\beta\beta$ decay experiment, CUORE (Cryogenic Underground Observatory for Rare Events). CUPID-Mo was operated at Laboratoire Souterrain de Modane in France as an array of 20 enriched Li$_{2}$$^{100}$MoO$_{4}$ (LMO) cylindrical crystals ($\sim$200g each), with each LMO featuring a Ge light detector (LD). The LMOs and LDs were operated at $\sim$20 mK and instrumented with NTD sensors allowing for the collection of both heat and scintillation light. This dual mode of energy collection allows for $\alpha$ events to be distinguished from $\beta$/$\gamma$ events, significantly reducing the background from degraded $\alpha$s in the heat channel. With a demonstrated bolometric energy resolution of $\sim$7 keV (FWHM) at 2615 keV, complete discrimination of $\alpha$'s from $\beta$/$\gamma$'s and very low radioactive contamination, CUPID-Mo is in the background-free regime. Here we present the current leading results of CUPID-Mo with an exposure of over 2 kg-years for the $0\nu\beta\beta$ decay of $^{100}$Mo. In this talk we present the analysis for this result, the current limit and the status of ongoing CUPID-Mo analyses.   

Sensitivity of the nEXO neutrinoless double beta decay experiment

The nEXO experiment is a proposed next-generation search for the neutrinoless double beta decay (NDBD) of $^{136}$Xe. The primary detector will be a 5-ton, monolithic liquid xenon TPC with a target enriched to 90\% in the isotope of interest. A detailed study of the expected sensitivity, completed in 2017, calculated the 90\% CL exclusion sensitivity on the NDBD half life to be $9.2\times10^{27}$ yrs, approximately two orders of magnitude beyond existing limits. In this talk, we will discuss a new evaluation of the sensitivity, given updates to the detector design and improved modeling of the signal readout. Specific improvements that have been made since the last publication include improved, data-driven modeling of signal development in the charge readout tiles (and subsequently improved modeling of the energy and position reconstruction), the development of new machine-learning analyses to improve signal/background separation, and an updated detector geometry which reflects changes made to the engineering design over the past three years.   

Latest Results from the Majorana Demonstrator

The MAJORANA DEMONSTRATOR is a Ge-based neutrinoless double-beta decay ($0\nu\beta\beta$) experiment currently operating at the 4850' level of the Sanford Underground Research Facility. The DEMONSTRATOR contains 44 kg of p-type point contact Ge detectors (30 kg enriched in Ge-76) in two modules that are surrounded by a low background passive shield. The experiment has a leading energy resolution of 2.5 keV FWHM as well as one of the lowest backgrounds in the region of interest surrounding the double-beta decay Q-value. The DEMONSTRATOR's high-resolution electronics and use of ultrapure materials to control backgrounds will form one component of LEGEND-200's approach to increasing sensitivity to $0\nu\beta\beta$. In this talk, I will review the MAJORANA DEMONSTRATOR's latest results after analysis improvements and discuss the recent performance of the array following a hardware upgrade.   

R&D Toward Ba-Tagging in nEXO Using Scanning Transmission Electron Microscopy

$^{136}$Xe has a unique capability as a neutrino-less double beta decay isotope to provide a near background-free measurement by identifying the remnant $^{136}Ba^{++}$ from the decay. This ''Ba-Tagging'' capability would increase the sensitivity of the nEXO experiment by a factor of 4, and allow nEXO to probe well into the normal mass hierarchy. Our group has shown that Scanning Transmission Electron Microscopes (STEM) have robust capability to image and identify single Ba atoms. However, work remains to develop the technical tools to extract the Ba ion from the LXe TPC and place it into the STEM for analysis, before this can become a practical technique. We present progress toward developing and testing a conceptual design for the entire Ba-tagging chain, from extraction of the single Ba ion out of the 5 tons of LXe to the end-stage identification in the STEM.   

Imaging of single Ba atoms and Ba$^{\mathrm{+}}$ ions in solid xenon for barium tagging in next-generation $^{\mathrm{136}}$Xe double beta decay experiments

The identification, or ``tagging'' of the barium-136 daughter atom that results from double beta decay of xenon-136 provides a promising technique for elimination of all backgrounds except 2-neutrino double beta decay in future generations of 136Xe neutrinoless double beta decay experiments. The Ba tagging scheme being developed utilizes a cryogenic probe to trap the 136Ba daughter atom in solid xenon and extract it from a liquid xenon time projection chamber, such as the nEXO design concept. The barium atom is then tagged via fluorescence imaging in the solid xenon matrix. The status of our efforts to image and count single Barium atoms and Ba$+$ ions in the solid xenon matrix will be presented. An important feature of the method is that any residual Ba atoms on the probe surface do not create an observable signal, only those that are captured in the solid xenon.   

Active Structural Materials for the Neutrinoless Double Beta Decay Experiment, LEGEND

With current half lives of neutrinoless double beta decay (0$\nu\beta\beta$) surpassing $10^{26}$ yrs, the next generation of experiments must diminish background values to increase experimental sensitivities. Backgrounds in these experiments should be reduced by pursuing both ultra-pure materials and efficient background identification with active vetos. Background identification can be further improved by replacing inactive structural components with transparent, radio-pure plastic scintillators. Poly(ethylene-2,6-naphthalate) (PEN) has been identified as a prospective material for structural scintillator components as it has strong mechanical properties, significant scintillation light yield in the 400 nm region. PEN structural components have been manufactured and their radioactivity and optical properties have been measured as a part of the LEGEND collaboration R&D program. This presentation will provide updates from this characterization and describe efforts being made to synthesize PEN for future tonne-scale 0$\nu\beta\beta$ experiments.