Studies of simulation framework for NνDEx experiment

TL;DR

This work develops a comprehensive simulation framework for the NνDEx high-pressure SeF$_6$ TPC to search for $0\nu\beta\beta$ decay in $^{82}$Se, incorporating dual-ion transport (SeF$_5^-$ and SeF$_6^-$) with a 20:80 yield ratio and computed reduced mobilities. It combines molecular modeling (DFT) and collision theory to obtain mobility parameters, COMSOL field calculations for a 10,000-pixel Topmetal-S readout, Geant4/BxDecay0 event generation, Garfield++ charge transport, and signal convolution with a realistic CSA transfer function. The analysis pipeline includes 3D track reconstruction via BFS, blob-based topology for double-electron signatures, six discriminating variables, and a Boosted Decision Tree that achieves 98.1% signal efficiency with 0.92% background, corresponding to a significance of 31.18. The framework supports design optimization and sensitivity studies, and the authors outline future work on gas parameter refinement, field optimization, and tracking enhancements to further improve background rejection and energy resolution. Overall, the framework demonstrates the viability of dual-ion charge collection for trigger-less, high-resolution $0\nu\beta\beta$ searches in a SeF$_6$-based TPC.

Abstract

The N$ν$DEx experiment aims to search for the neutrinoless double beta decay of $^{82}$Se using a high-pressure $^{82}$SeF$_6$ gas time projection chamber (TPC). Under the assumption of two kinds of charge carriers would be formed, the difference in drift velocities between these ion species enables trigger-less event reconstruction and offers the potential for excellent energy resolution through direct charge collection. In this study, we present a simulation framework for the N$ν$DEx ion TPC. The reduced mobilities of SeF$_5^-$ and SeF$_6^-$ ions in SeF$_6$ were calculated using density functional theory and two-temperature theory, yielding values of $0.444 \pm 0.133$ and $0.430 \pm 0.129$ cm$^2$V$^{-1}$s$^{-1}$, respectively. The TPC geometry, featuring a cathode-focusing plane-anode structure and an 10,000-pixel readout array, was modeled with COMSOL to calculate the electric and weighting fields. Signal and background events were generated using BxDecay0 and Geant4. Garfield++ was used to simulate the transport of charge carriers and signal induction. The induced current was convolved with the transfer function to produce voltage signals, which were subsequently used to extract energy through amplitude. The 3D tracks are also reconstructed based on drift time differences and Breadth First search. To enhance signal background separation, six topological variables were extracted from the reconstructed tracks and used to define optimized selection criteria. The Boosted Decision Trees is used for a preliminary analysis. This simulation framework serves as a crucial tool for design optimization and sensitivity studies in the N$ν$DEx experiment.

Figures (22)

• Figure 1: The simulation and analysis framework mainly consists of four components: the event generator, particle transportation, detector response, and analysis algorithms.
• Figure 2: The molecule model of SeF$_5^-$ (left) and SeF$_6^-$ (right) after optimization by Avogadro.
• Figure 3: Comparison between simulated and test results: Left: drift behavior of charge carriers SF$_5^-$ and SF$_6^-$ in SF$_6$ gas, showing the physics transportation of what happened in TPC; Right: output voltage waveform of Topmetal-S readout from test pulse measurements, used for validating the simulation of front-end electronics response.
• Figure 4: Structural diagram of the unit cell simulated in COMSOL. The electric field lines are also shown, with the color representing the field strength.
• Figure 5: Cross-sectional view of the weighting potential near the sensor array.