Session E14: Neutrinoless Double Beta Decay II

Search for Majoron-emitting modes of $^{136}$Xe double beta decay in EXO-200

EXO-200 was a single phase liquid xenon Time Projection Chamber (TPC) detector located at  the Waste Isolation Pilot Plant. The experiment collected data taking in two phases between May 2011 and December 2018. EXO-200 demonstrated good energy resolution and excellent background discrimination in an ultra-low background TPC. A search for Majoron-emitting modes of the neutrinoless double-beta decay of $^{136}$Xe is performed with the full EXO-200 dataset. This dataset consists of a total $^{136}$Xe exposure of 234.1 kg$\cdot$yr, and includes data with detector upgrades that improved the energy threshold relative to previous searches. In this talk, we present the the analysis methods and results of searching for Majoron-emitting modes of $^{136}$Xe double beta decay with the complete EXO-200 dataset.    

Simulation of charge reconstruction in the nEXO experiment

The nEXO experiments aims to employ a tonne scale liquid xenon time projection chamber to search for the hypothetical neutrinoless double beta decay (0vbb) of Xe-136, which can only occur if neutrinos are Majorana fermions. Recent studies of the proposed nEXO detector design indicate that it will enable a search for this interaction with a half-life sensitivity greater than $10^{28}$ yrs. In order to achieve this goal, nEXO must collect and reconstruct the charge deposited by candidate 0vbb events with sub-percent accuracy. This talk will summarize the simulation of charge collection in nEXO and the algorithms being developed for optimally reconstructing the energy and location of charge deposits in the detector.   

Device testing of VUV SiPMs for nEXO

The tonne-scale nEXO experiment aims to search for neutrinoless double beta decay with a half-life sensitivity of ~10²⁸ years. The nEXO detector is a liquid xenon time projection chamber enriched to 90% ¹³⁶Xe that uses vacuum ultraviolet (VUV) silicon photomultipliers (SiPMs) as scintillation sensors. This talk will cover the testing of the SiPM devices that are under consideration for use in nEXO. We present the characterization methods and results, focusing on the photon detection efficiency (PDE), single photoelectron (SPE) gain, temperature-dependent breakdown voltage, dark count rate, and correlated avalanche contribution.    

Search for Two-Neutrino Decays of 130Te to Excited States With CUORE

The CUORE experiment is a ton-scale search for neutrinoless double-beta decay (0νββ) composed of an array of 988 tellurium dioxide crystals, each instrumented as a cryogenic macrocalorimeter. While a discovery of 0νββ would herald new physics, the Standard Model process of two-neutrino double-beta decay (2νββ) in ¹³⁰Te is readily measured by CUORE. However, still unobserved in ¹³⁰Te are decays wherein the daughter ^{​​​​​​​130}​​​​​​​Xe nucleus is left in an excited state which subsequently decays via gamma emission, leading to events with energy deposited across multiple crystals. Understanding the branching ratio and spectral shape for such 2νββ decays to excited states can improve nuclear modelling and help constrain the matrix elements involved in searches for neutrinoless double-beta decay. Progress is ongoing towards an improved search for 2νββ decays to excited states with an increased exposure of CUORE. In this talk, I will describe some of the analysis techniques to be implemented in the search, along with their expected impact on discovery sensitivity.   

Denoising Algorithms for the CUORE Experiment

The Cryogenic Underground Observatory for Rare Events (CUORE) experiment is an ongoing search for neutrinoless double beta decay located at the Gran Sasso National Laboratory (LNGS) in Italy. Our previous work has suggested that the quality of CUORE data can be improved by the use of noise decorrelation algorithms using data from auxiliary devices included microphones and accelerometers.  Here I will present the results of these decorrelation algorithm and how it impacts the energy resolution of the CUORE detector across multiple channels. I will also discuss how these denoising algorithms can be expanded to model non-linear systems and how these expansions improve the performance of the aforementioned noise decorrelation algorithms for the CUORE detector.   

Detector Response to Seismic Activity in CUORE

The Cryogenic Underground Observatory for Rare Events (CUORE) is a ton-scale detector searching for neutrinoless double beta (0νββ) decay in the Gran Sasso National Laboratory (LNGS), Italy. The experiment is an array of 988 TeO2 crystals, each of which is a highly sensitive calorimeter detecting temperature fluctuations at the millikelvin level arising from energy depositions. The excellent energy resolution of the calorimeters is critical to the search for 0νββ and other rare events. In this talk, we discuss studies of correlations between seismic activity and increased noise in the detector, which may contribute to degraded energy resolution.   

Progress in Background Model Fitting for the MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR is a neutrinoless double-beta decay experiment consisting of two modules containing ~30kg of p-type point contact germanium detectors enriched to 88% in 76Ge and ~14kg of natural germanium detectors.  These modules are surrounded by a graded shield and located at the 4850' level of the Sanford Underground Research Facility. The DEMONSTRATOR has achieved one of the lowest background rates in the region of the 0?????? Q-value. In addition, extremely low backgrounds in the energy region below 100 keV 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, explain deviations from assay-based projections, and allow a precision measurement of the two-neutrino double-beta decay half-life.  In this talk, we present recent progress in background modeling including improved fits to a significant portion of the experiment's full exposure and efforts to quantify systematic errors due to model uncertainties.   

Modeling 232Th Backgrounds in the MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR is a neutrinoless double beta decay (0νββ) experiment consisting of ~30 kg of germanium detectors enriched to 88% in ⁷⁶Ge and ~14 kg of natural germanium detectors. The detectors are divided between two cryostats and surrounded by a graded passive shield. The measured background of 11.9 ± 2.0 cts/(FWHM t y) at the double beta decay Q-value is among the lowest background rates in 0νββ experiments but is still more than a factor of four higher than the projected background rate based on component assays. This discrepancy arises from an excess of events from the ²³²Th decay chain. The location(s) of the excess can potentially be revealed by comparing data to simulations through studies of the relative intensities of high and low energy peaks, comparisons of peak rates between detectors, and background model fits. Indications that near-detector locations are not consistent with the observed background have informed design decisions for components being used in the next-generation LEGEND experiment. More specific information on the source of the excess could provide additional input to LEGEND and other low-background experiments. Recent findings have identified a few suspected locations for the excess activity, motivating additional simulations and assays.   

Isotopic signatures and neutron simulations for LEGEND using Geant4 and MCNP

The ton-scale LEGEND project will search for neutrinoless double-beta decay in ⁷⁶Ge. The first phase of the project, LEGEND-200, is in commissioning at the Laboratori Nazionali del Gran Sasso. To assess the suitability of the candidate host sites for the final LEGEND-1000 phase of the project, an estimate of the background induced by muons is necessary. Simulations of these backgrounds, particularly those via muon-induced neutrons, come with large uncertainties. Benchmarking simulations using Geant4 and MCNP are being executed and analyzed to assess systematic uncertainties. A search for key radioactive isotopes produced by neutron captures will be performed in LEGEND-200, which can be used to estimate the in-situ cosmogenic background in LEGEND phases. This talk discusses the MCNP-Geant4 comparisons, identifies isotopic signatures which might be visible in LEGEND-200, and uses simulations to estimate the rate for these signatures.   

Transition edge-sensor multiplexing readout for CUPID-1T

The CUPID (CUORE with Upgraded Particle IDentification) experiment is a next-generation double beta decay search experiment using cryogenic calorimeters and light detectors to measure heat and light, respectively, released by an interaction. The dual channel detection allows for particle identification and thus for background rejection. However, the superimposition of events close in time, referred to as pileup, remains one of the major background contributions, in particular the one arising from the two-neutrino double beta decay process. In view of further detector upgrades, as planned for CUPID-1T, fast detectors are required to be able to discriminate this source of background and transition-edge sensors are one of the candidates that satisfy the timing requirements needed for it. To reduce the heat load coming from the foreseen ten thousand channels or more into the cryostat, whose working point is at about ten mK, and to minimize the radioactive materials around it, multiplexed readout is necessary in this configuration. This technique is being investigated in collaboration with the Lawrence Berkeley National Laboratory and UC Berkeley. The current status of the project will be presented.