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A REST-based Monte Carlo Simulation and Data Analysis Framework for the N $\nu$ DEx Experiment

This paper presents a comprehensive REST-based simulation and reconstruction framework for the N $\nu$ DEx experiment, which successfully models the detector's response to achieve a sub-1% energy resolution, thereby establishing a critical foundation for future background rejection and design optimization studies.

Original authors: Siyuan Huang, Tao Li, Hao Qiu, Shaobo Wang

Published 2026-10-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Siyuan Huang, Tao Li, Hao Qiu, Shaobo Wang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the Earth, beneath the towering mountains of western China, scientists are preparing to listen for a whisper that has eluded humanity for nearly eighty years. They are hunting for a ghostly event called neutrinoless double-beta decay. To understand why this matters, one must first accept a strange possibility about the universe: that the neutrino, a tiny particle that passes through everything without leaving a trace, might be its own antiparticle. If this is true, it would rewrite the rules of physics, explaining why the universe is made of matter rather than being an empty void of equal parts matter and antimatter. While standard physics predicts that certain heavy atoms should occasionally split into two protons and two electrons while releasing two invisible particles called antineutrinos, the special version of this decay the scientists seek would release only the two electrons and nothing else. Finding this rare event would prove that neutrinos are indeed their own antiparticles, a discovery that would unlock secrets about the fundamental nature of mass and the origin of the cosmos.

The NνDEx experiment is the latest and most ambitious attempt to catch this elusive signal. It is designed to watch over a specific type of atom, selenium-82, waiting for one of its nuclei to undergo this forbidden transformation. The experiment will take place in the China Jinping Underground Laboratory, a facility buried deep beneath rock to shield it from cosmic rays and other background noise that could drown out the faint signal. The heart of the detector is a large, high-pressure tank filled with a gas made of selenium and fluorine. When a decay happens, it leaves behind a trail of ionized gas, much like a plane leaves a vapor trail in the sky. The challenge is that these trails are incredibly faint and easily confused with noise from natural radioactivity. To solve this, the researchers need a way to see the shape of the trail with perfect clarity, distinguishing the unique signature of a double-beta decay from the messy tracks left by ordinary background radiation.

In this work, a team of researchers has built a complete digital twin of the NνDEx detector to test if their design will actually work before the physical machine is even finished. They used a sophisticated software framework called REST, which is designed to simulate how particles move and interact inside complex detectors. Instead of building the machine first and hoping for the best, they created a virtual version where they could generate millions of fake decay events and watch how the detector would respond. They programmed the software to mimic every physical detail of the real device, from the thick layers of lead and copper that shield the tank to the specific way the gas molecules capture electrons and turn them into negative ions. They even simulated the drift of these ions through the gas, the way they spread out over time, and how they finally hit a grid of tiny sensors at the end of the tank.

The simulation revealed that the detector's design is capable of capturing the signal with remarkable precision. The researchers found that the gas inside the tank behaves in a very specific way: when an atom decays, it releases electrons that are quickly grabbed by gas molecules to form two different types of heavy ions. Because these two types of ions move at slightly different speeds, they arrive at the sensor at different times, creating a distinct double-peak signal. This timing difference acts like a ruler, allowing the computer to calculate exactly how far the ions traveled and where the decay happened. By analyzing the shape of the electrical signal produced when these ions hit the sensor, the team developed a method to reconstruct the original path of the decay with high accuracy. They tested this method on simulated data and found that it could recover the energy of the event with an error of less than one percent.

The results of this simulation are a strong vote of confidence for the NνDEx project. The digital tests showed that the detector can distinguish the clean, sharp tracks of a potential neutrinoless double-beta decay from the chaotic, messy tracks of background noise. The energy resolution achieved in the simulation, a measure of how precisely the machine can measure the energy of a particle, reached a level of 0.58 percent. This is well within the strict requirements needed to spot the rare decay if it exists. The researchers also determined the best settings for filtering out electronic noise, ensuring that only the most significant signals are analyzed. By proving that their software can successfully track these events from the moment they occur in the gas to the final digital readout, the team has laid a solid foundation for the future construction of the experiment.

This work does not yet prove that neutrinoless double-beta decay exists; that discovery will only come when the real detector is built and begins collecting data. However, the simulation confirms that the NνDEx experiment is built on a sound scientific foundation. It demonstrates that the combination of high-pressure gas, precise sensors, and advanced data analysis can create a machine sensitive enough to hear the faintest whisper of new physics. The path from a theoretical idea to a working detector is long and difficult, but this study shows that the blueprint is correct. If nature is kind, and the decay happens, the NνDEx experiment will be ready to find it, potentially answering one of the most profound questions about the universe: why we are here at all.

Technical Summary: A REST-based Monte Carlo Simulation and Data Analysis Framework for the NνDEx Experiment

Problem Statement
The NνDEx experiment aims to search for neutrinoless double-beta decay ( $0\nu\beta\beta$ ) of $^{82}\text{Se}$ using a high-pressure gaseous time projection chamber (TPC) filled with $^{82}\text{SeF}_6$ gas and a Topmetal pixelated charge readout system. While the high $Q_{\beta\beta}$ value of $^{82}\text{Se}$ (2.996 MeV) and the track reconstruction capabilities of a gaseous TPC offer strong potential for background suppression, realizing this potential requires a precise understanding of the detector's response. Specifically, the experiment must model complex physical processes, including negative-ion drift and diffusion in electronegative gas, the mapping of drifting ions to a segmented readout plane, and the formation of electronic waveforms. Prior to this work, a complete software description linking energy deposition to digitized waveforms, tailored to the specific geometry and physics of the NνDEx detector, was needed to validate design requirements and optimize reconstruction algorithms.

Methodology
The authors developed a full simulation and reconstruction framework based on the REST (Rare Event Searches Toolkit) software. The workflow is divided into simulation and reconstruction chains:

Simulation Chain:
- Geometry: A detailed geometric model of the NνDEx-100 detector was constructed, including external shielding (HDPE and lead), a high-pressure stainless-steel vessel, an inner oxygen-free copper shield, and the active $^{82}\text{SeF}_6$ gas volume.
- Event Generation: Primary events ( $0\nu\beta\beta$ decay of $^{82}\text{Se}$ ) were generated using the DECAY0 generator and transported via Geant4. Energy deposits were converted into ionization electrons using a Fano-factor-sampled Gaussian distribution ( $W=32$ eV, $F=0.19$ ).
- Charge Transport: Recognizing that $^{82}\text{SeF}_6$ is electronegative, the simulation modeled charge transport via negative ions (primarily $\text{SeF}_5^-$ and $\text{SeF}_6^-$ ) rather than free electrons. The framework calculated drift velocities and longitudinal/transverse diffusion based on reduced mobility, electric field strength, pressure, and temperature.
- Readout and Electronics: The simulation mapped drifting ions to a hexagonal grid of 8,192 readout cells (32 modules $\times$ 16 $\times$ 16 cells). The arrival time distribution of ions was convolved with a charge-sensitive amplifier response function (rise time $\approx$ 50 ms) to generate waveforms. Electronic noise (RMS $\approx$ 45 $e^-$ ) was added, resulting in digitized waveforms with a 20-second acquisition window.
Reconstruction Chain:
- Waveform Fitting: A waveform-fitting method was applied to extract deposited charge and event energy. The fitting function was constructed by convolving a double-Gaussian ion-arrival distribution (representing $\text{SeF}_5^-$ and $\text{SeF}_6^-$ ) with the electronics response function.
- Optimization: To mitigate noise, a signal threshold ( $S_t = 225$ $e^-$ or 7.5 mV) and a low-pass filter (cutoff frequency $f_c = 50$ Hz) were optimized. These parameters were selected to balance waveform smoothness for stable initial value estimation against the preservation of peak structures necessary for accurate drift distance and energy reconstruction.
- Energy Reconstruction: The total event energy was obtained by summing the reconstructed energies of individual ionization points. The reconstructed spectrum was fitted using a Crystal Ball function.

Key Contributions

Customized REST Implementation: The paper presents the first full simulation and reconstruction chain for the NνDEx experiment within the REST framework, specifically implementing the unique physics of negative-ion drift in $^{82}\text{SeF}_6$ and the Topmetal pixelated readout.
Physics Modeling: The framework accurately models the double-peak structure in readout waveforms caused by the different drift velocities of $\text{SeF}_5^-$ and $\text{SeF}_6^-$ ions, which provides timing information for drift distance reconstruction.
Algorithm Development: A robust waveform-fitting algorithm was developed and optimized to handle the specific noise characteristics and signal shapes of the NνDEx detector, including the determination of optimal filtering and thresholding parameters.

Results

Energy Resolution: For simulated $0\nu\beta\beta$ signal events, the reconstructed energy spectrum achieved a Full Width at Half Maximum (FWHM) energy resolution of 0.58%.
Comparison to Design: This result satisfies the experiment's design requirement of less than 1% FWHM. It is consistent with the theoretical estimate of 0.61% and represents a slight degradation from the intrinsic 0.33% resolution of the simulated energy deposition, attributed to reconstruction effects.
Reconstruction Accuracy: With the optimized parameters, the reconstruction deviations for energy and drift distance were kept below 4% and 5%, respectively, for signals above the noise threshold.

Significance
The authors state that this simulation and reconstruction chain provides a foundational basis for future studies within the NνDEx program. Specifically, the framework enables:

Detailed studies of detector response.
Development of background rejection strategies.
Event classification algorithms.
Optimization of the NνDEx detector design.

The work demonstrates that the proposed detector concept, when coupled with the developed software tools, can meet the stringent energy resolution requirements necessary for a competitive search for neutrinoless double-beta decay.

Technical Summary: A REST-based Monte Carlo Simulation and Data Analysis Framework for the NνDEx Experiment

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