Search for Double Beta Plus Decays with a Novel Detector Concept Combining Hybrid and Opaque Scintillator Techniques

The pre-dominance of ordinary matter over anti-matter in the observable universe is one of the major unsolved questions of cosmology. Without this so-called baryon asymmetry (BA) the universe would be a void. An avenue to explain BA is through the discovery of a process that violates the number of leptons (LNV). Prime candidate is the neutrinoless double beta decay (0n2b). While in ordinary double beta decays (2n2b) two nucleons simultaneously decay in two charged leptons and two neutrinos, in 0n2b the neutrinos are internally absorbed leading to LNV. In most theories this is connected to the notion of Majorana fermions (MF), i.e. the neutrino being its own anti-matter partner. Neither LNV nor MF have been observed yet. Thus discovery of 0n2b would revolutionise our understanding of the universe.Currently, searches focus on negative double beta decays. In contrast, double beta plus (2b+), double electron capture (2EC) and their mixed mode (ECb+) are little explored. None of the positive neutrinoless modes and not even 2n2b+ nor 2nECb+ have been seen although the latter two are undisputedly expected to exist. Their measurement is particularly difficult due to suppressed decay probabilities, challenging background suppression and low natural abundances of suitable candidate nuclei. In this application I propose NuDoubt, a novel detector concept based on the combination of hybrid and opaque scintillation detector technology. They each offer groundbreaking background suppression. NuDoubt allows for the first time to combine their complementing background suppression power achieving superb signal to background ratios. NuDoubt is perfectly suited for pressure loading of enriched noble gases allowing for unmatched abundances in scintillation detectors. NuDoubt will measure for the first time 2nECb+ within just 1 calendar year of operation. NuDoubt will also be able to probe 0n2b+ and 0nECb+ at more than 1000-fold improved significance compare to current limits.