Session D12: Minisymposium: Low Energy Neutrinos II: Neutrinoless Double-Beta Decay II

Test and predict neutrinoless double-beta decay with other observables

Predictions of neutrinoless double-beta decay (0nbb) nuclear matrix

elements are challenging, and markedly differ from each other when

different many-body methods are used. One possible avenue to improve

this current status is to use data from other observables related to

0nbb to test the theoretical calculations, and to use correlations of

these observables with 0nbb matrix elements to predict them more

confidently. In my talk I will present recent work focused on using

nuclear spectroscopy data to test calculations and double-gamma and

two-neutrino double-beta decays to predict 0nbb nuclear matrix

elements.   

Comprehensive Review of Double-Beta Decay Half-Lives

The double-beta decay is the rarest nuclear physics process, and its experimental half-lives (T$_{1/2}$) exceed the age of the Universe from nine to fourteen orders of magnitude.  Double-beta decay was observed, and its half-life was measured in 14 parent nuclei using direct and geochemical methods.  The decay observables are analyzed using the Evaluated Nuclear Structure Data File (ENSDF) procedures,  and the recommended T$_{1/2}$ were deduced.

Using the calculated values of phase factors,  the effective nuclear matrix elements were extracted and compared with available data. Many thousands of theoretical and experimental works are dedicated to these topics in the last 85 years, and we present two data sets of recommended values to encapsulate the results.   

Reduction of 212Bi background in CANDLES experiment

The neutrino-less double beta decay (0νββ) is a process that occurs only when neutrinos and antineutrinos are identical (Majorana). This process is very important for the determination of Majorana nature and absolute scale of neutrino mass, but it has not yet been observed. In the 0νββ decay search experiment, the decay rate is quite rare thus it is required to be extremely reduced the background events (BG) near the Q-value of 0νββ.

In the CANDLES experiment, 96 CaF2 scintillation crystals with natural Ca (containing 350 g of ⁴⁸Ca) were used to search for 0νββ. The Q-value of ⁴⁸Ca is 4.27 MeV, which is highest among double beta decay isotopes, so that BG rate is strongly suppressed. 

  One of the major BGs is the pile up waveform, The half-life of ²¹²Po beta decay(T_1/2 = 299 nsec) is shorter than the decay-time of CaF2 scintillation (1 μsec); hence, the delayed ²¹²Po α-decay piles up the prompt ²¹²Bi β decay (²¹²BiPo event).

The ²¹²BiPo event is observed as one event when the time-lag of the decays is relatively short (Double Pulse: DP). 

This ²¹²BiPo events are difficult to distinguish from the waveform of double beta decay (Single Pulse: SP) by conventional fitting, which reduces the sensitivity to 0νββ.

We developed the new analysis tool using machine learning to identify this pile up waveform and evaluated the BG reduction rate. The removal efficiency of ²¹²BiPo event by discriminating SP and DP waveforms using machine learning is presented.   

Pulse-Shape-Based Analysis with Recurrent Neural Networks in the MAJORANA DEMONSTRATOR

The Majorana Demonstrator experiment was a search for neutrinoless double-beta decay (0νββ) in ⁷⁶Ge using p-type point contact high-purity germanium detectors. The data-taking for 0νββ by the DEMONSTRATOR has successfully completed in March 2021 and set a 0νββ half-life limit of T_1/2 > 8.3´10²⁵ yrs based on its full exposure. The MAJORANA Collaboration developed traditional pulse-shape-based approaches to discriminate different types of events, such as multi-site (MS) events and single-site (SS) events. The collaboration is also exploring machine learning (ML) tools for event discrimination, such as recurrent neural networks (RNN). In this talk, we will discuss MAJORANA ML efforts. For example, the RNN is used to tag pileup-event waveforms, due to both random coincidences in calibration data and real physics correlations. The talk will also discuss the interpretable ML models for SS and MS event discrimination where the attention mechanism is implemented to focus on the most important component of the waveform, and to enhance the network performance.   

Improving Pulse Shape Simulations with Generative Adversarial Machine Learning

The Large Enriched Germanium Experiment for Neutrinoless double-beta Decay (LEGEND) collaboration is searching for neutrinoless double-beta 0νββ decay in ⁷⁶Ge using modular arrays of germanium detectors enriched in the isotope. Candidate 0νββ events happen at a single site in the germanium detector. Pulse shape simulations to model the movement of charge carriers in the detectors are key to accurately modelling requirements that can reject background from multi-site and surface events. However, a series of corrections based on physical first principles are needed to correctly generate an accurate detector response. We present a novel neural network model called Cyclic Positional U-Net (CPU-Net) that enables translations of simulated pulses such that they are indistinguishable from actual detector pulses. The model uses a CycleGAN framework to develop an Ad-hoc Translation Network (ATN) for such translations with high precision and low latency. Using an HPGe detector, we demonstrate that the model correctly reproduces critical pulse shape reconstruction parameters, and that its data-driven nature enables generalization to multiple detectors and operating conditions without detector-wise model tuning.   

Machine Learning-Powered Data Cleaning for LEGEND

The Large Enriched Germanium Experiment for Neutrinoless Double-Beta Decay (LEGEND) will operate in two phases in the search for neutrinoless double-beta decay (0νββ). The first (second) stage will employ up to 200 (1000) kg of ⁷⁶Ge semiconductor detectors to achieve a half-life sensitivity of 10²⁷ (10²⁸) years. In this study, we present a data-driven approach to remove non-physical events captured by ⁷⁶Ge detectors in LEGEND-200 powered by a novel artificial intelligence model. We utilize Affinity Propagation to cluster events based on their shape and a Support Vector Machine to classify events into different categories. We demonstrate that our model efficiently classifies different categories of events, achieving a physical event sacrifice of < 0.001 %. This method will provide an automated data cleaning mechanism for LEGEND, which requires significant time and human effort when performed with traditional procedures.   

100Mo Neutron Activation Background Measurement for CUPID

CUPID, a proposed next-generation ton-scale neutrinoless double beta decay search, will deploy an array of ~1600 Li2Mo04 bolometers operated near 10mK. 100Mo, the decaying isotope of interest, offers multiple benefits in its enriched crystal LiMoO4 form, one of which is the ability to reject backgrounds due to alpha contamination with a secondary scintillation signal. Deploying a massive detector of 100Mo also necessitates a search for any potential backgrounds associated with the isotope, in particular neutron activated backgrounds. I will discuss gamma spectroscopy measurements taken at Duke's Tandem Facility, where enriched 100Mo was exposed with 4-6MeV neutrons.   

Photo-induced Charge Calibration R&D for nEXO

The 5-ton liquid xenon TPC of the nEXO experiment is designed to search for the elusive neutrinoless double beta decay of ¹³⁶Xe with a half-life sensitivity goal of >10²⁸ years. The challenging task of calibrating the detector, a monolithic 1.3 m diameter right cylinder involves the regular deployment of external radioactive sources and the occasional injection of  ²²⁰Rn to understand the ionization and light response. Currently, there are several ongoing R&D efforts to incorporate additional calibration techniques to mitigate risk and to regularly monitor the drift electrons and the light response of silicon photomultipliers in liquid xenon. This presentation will explore the status and preliminary results from an ongoing project at the University of Massachusetts. The goal of this project is developing an optimal strategy to use multiple gold photocathodes to generate photoelectrons and to monitor the lifetime of the drifting electrons quasi-continuously in-situ in the TPC   

Light detection and simulation for nEXO

nEXO is a 5 tonne liquid xenon time projection chamber (TPC) that

aims to detect neutrinoless double beta decay in ¹³⁶Xe with a projected

90% CL half-life sensitivity of 1.35x10²⁸ yr. nEXO will be able to mea-

sure energy deposits from both ionization electrons and scintillation pho-

tons from events that occur inside the detector. Silicon photomultiplers

(SiPMs) will surround the sides of the TPC to detect scintillation light,

providing a more highly pixellated light collection system than most pre-

vious liquid Xe TPCs. With this design, the detector will have high light

collection efficiency and energy resolution of σ/Q_ββ < 1% as well as the

ability to image the location of interactions based on their light signals.

This talk presents the current characterization of candidate SiPMs and

simulation of the light response in the nEXO detector.   

Synthesis and Applications of Low Background Modified Polyethylene Naphthalate (PEN-G)

Identification of background radiation is of utmost importance for enabling rare event experiments to attain the required sensitivities for probing new physics. Poly(ethylene-2,6-naphthalate) (PEN) has emerged as a highly promising material for such experiments due to its intrinsic scintillating properties and its adaptability as a structural material at both room and cryogenic temperatures. Notably, PEN has been successfully implemented in the LEGEND-200 experiment involving the target isotope $^{76}$Ge for investigating neutrinoless double beta decay. In LEGEND-200, PEN serves as both an active material and a structural component within the detector assembly. Looking towards the next-generation experiment, LEGEND-1000 will further reduce background radiation by an order of magnitude. To achieve this goal, we are looking to expand more potential applications of PEN-G. To this end, we have successfully synthesized PEN in kilogram batches utilizing unique reagents. The radiopurity of the synthesized PEN has been measured, and we are exploring strategies to improve these values. In this presentation, we will outline the methods employed and present the corresponding results.   

Characterizing the Temperature Dependence of Charge Trapping Effects in HPGe Detectors for use in Neutrinoless Double-Beta Decay Experiments

An effect which can have a significant impact on charge collection and signal readout in semiconductor detectors is `charge trapping,' whereby defects in the crystal (e.g. dopant impurities or dislocations in the lattice) cause localized electronic states in the band-gap which can cause charge carriers to weakly bind to them, temporarily trapping them until they are released back into the conduction band. The characteristic trap time, τ<sub>ct</sub>, depends on the effective `depth' of the trap and the temperature of the system; at low temperatures, these release times can become long compared to the signal collection time, leading to degraded signal amplitude and an inability to reconstruct the `true' energy of the initial event.

For germanium-based 0νββ experiments such as LEGEND (the Large Germanium Experiment for Neutrinoless double-beta Decay), understanding the nature and origin of these trapping sites in HPGe detectors is critical, as they require the ability to reconstruct the energy deposition and the topology of particle interactions in their detectors to an extreme degree. This talk will present an overview of an experimental test stand being developed at Los Alamos National Laboratory to identify the nature of the charge trapping sites in the HPGe crystals using temperature-dependent measurements of the defect-specific characteristic release times τ_ct.   

Development of Germanium (Ge) Ring Contact Detectors for Ge-based Neutrinoless Double-Beta Decay Experiment

The next generation neutrinoless double beta (0νββ) decay experiments aim to achieve sensitive to a decay with a half-life of ~10²⁸ years. A germanium-76 (Ge-76)-based experiment can achieve the discovery potential for this rare decay process due to its excellent energy resolution and ability to reject scattered gamma-ray events.  LEGEND-1000 prefers large-size detectors (>3 kg per detector) to further reduce backgrounds, complexity, and cost. This talk will explore large-size Ge fabricated in a novel ring contact (GeRC) geometry using high-purity Ge crystals grown at USD. The GeRC detector, a collaborative effort between ORNL, UNC, TAMU, and USD, has undergone significant improvements since its inception. Last year, an initial GeRC detector has been successfully fabricated at TAMU and subsequently tested at UNC. Regrettably, this first iteration did not meet our expectations. However, this presents an opportunity to enhance our research capabilities. By establishing an upgraded workshop at USD, we are now better equipped to study GeRC detectors comprehensively. Leveraging the invaluable experience gained from TAMU, we are poised to refabricate the first detector at USD. In this presentation, we will present some preliminary results obtained from the GeRC detector fabricated at USD, utilizing crystals grown exclusively at USD.