Search for Double Beta Plus Decays with NuDoubt++

TL;DR

The paper addresses whether neutrinos are Majorana particles by probing lepton-number-violating decays, focusing on positron-emitting channels such as $2\\nu2\\beta^+$ and $0\\nu2\\beta^+$. It proposes NuDoubt++, a detector concept that combines hybrid slow scintillators, opaque scintillators, optimised light guides (OWL-fibres), and isotope loading to enable first measurements of positron-emitting double weak decays with high background rejection. Key contributions include a fully integrated detection approach, validated by table-top prototypes, capable of measuring $2\\nu2\\beta^+$ within ~1 tonne-week exposure and achieving ~3 orders of magnitude improvement in $0\\nu2\\beta^+$ sensitivity, across multiple isotope loadings (Kr-78, Xe-124, Cd-106, BaSO4). The work offers a practical path to exploring Majorana neutrinos and lepton-number violation with enhanced experimental reach, potentially opening a new sector in positron-emitting double-weak decays within the next several years.

Abstract

Double beta plus decay is a rare nuclear disintegration process. Difficulties in its measurement arise from suppressed decay probabilities, experimentally challenging decay signatures and low natural abundances of suitable candidate nuclei. In this presentation, we propose NuDoubt++, a new detector concept to overcome these challenges. It is based on the first-time combination of hybrid and opaque scintillation detector technology paired with novel light read-out techniques. This approach is particularly suitable detecting positron (beta plus) signatures. We expect to discover two-neutrino double beta plus decay modes within 1 tonne-week exposure and are able to probe neutrinoless double beta plus decays at several orders of magnitude improved significance compared to current experimental limits.

Figures (1)

• Figure 1: Illustration of the proposed strategy of particle discrimination in opaque scintillator based on the morphology of hits of optical photons per fibre. Geant4 Geant4_2003 simulations of (\ref{['fig:subfig_a']}) an electron, (\ref{['fig:subfig_b']}) a gamma-ray, (\ref{['fig:subfig_c']}) a single positron, and (\ref{['fig:subfig_d']}) two positrons. All four events deposit a total energy of 2.8 MeV in the detector. Additionally, the ratio of hits of Cherenkov and scintillation photons for each fibre is displayed (\ref{['fig:CSsubfig_a']}, \ref{['fig:CSsubfig_b']}, \ref{['fig:CSsubfig_c']}, and \ref{['fig:CSsubfig_d']}) for the same four events. Hit fibres with $\text{\v{C}/S} = 0$ are represented in grey on the colour scale. Fibres without hits have white colour. Figures reproduced from NuDoubt_2025 under http://creativecommons.org/licenses/by/4.0/.