Abstract
There are valuable arguments to perform neutrinoless double beta (0nu 2beta ) decay experiments with several nuclei: the uncertainty of nuclear-matrix-element calculations; the possibility to test these calculations by using the ratio of the measured lifetimes; the unpredictability of possible breakthroughs in the detection technique; the difficulty to foresee background in 0nu 2beta decay searches; the limited amount of isotopically enriched materials. We propose therefore approaches to estimate the Majorana neutrino mass by combining experimental data collected with different 0nu 2beta decay candidates. In particular, we apply our methods to a next-generation experiment based on scintillating and Cherenkov-radiation bolometers. Current results indicate that this technology can effectively study up to four different isotopes simultaneously (^{82}Se, ^{100}Mo, ^{116}Cd and ^{130}Te), embedded in detectors which share the same concepts and environment. We show that the combined information on the Majorana neutrino mass extracted from a multi-candidate bolometric experiment is competitive with that achievable with a single isotope, once that the cryogenic experimental volume is fixed. The remarkable conceptual and technical advantages of a multi-isotope investigation are discussed. This approach can be naturally applied to the proposed CUPID project, follow-up of the CUORE experiment that is currently taking data in the Gran Sasso underground laboratory.
Highlights
The exploration of the inverted neutrino mass hierarchy is a challenge of the experiments in preparation or R&D stage
Let us consider the mass mechanism for light neutrino exchange, which leads to the following expression for the half-life T10/ν2 of the nuclei candidates to the 0ν2β decay: the method of sum of counts has a drawback in the case of different detectors performance: if all the Si values are close to 0, the biggest contribution will be given by the measurement with the highest background
Two approaches to estimate the Majorana neutrino mass from 0ν2β decay experiments with several nuclei are proposed. The former method uses a sum of counts in the region of interest, while the latter one estimates the Majorana neutrino mass from weighted averages and errors of neutrino mass squares
Summary
The exploration of the inverted neutrino mass hierarchy ( mν ∼ 0.02–0.05 eV) is a challenge of the experiments in preparation or R&D stage. Taking into account the possible quenching of the axial vector coupling constant gA [19,20,21,22], the experimental program should foresee the possibility to go towards an-order-of-magnitude higher half-life sensitivities, of the order of T1/2 ∼ 1027–1028 yr The accomplishment of such an ambitious plan requires the construction of detectors containing a large number of 2β active nuclei (1027–1028 nuclei, ∼ 103–104 moles of isotope of interest), with an ashigh-as-possible detection efficiency and an extremely low (ideally zero) radioactive background, and able to distinguish the effect searched for. C (2018) 78:272 group of interest [30] proposes to perform a tonne-scale bolometric 0ν2β decay search with a sensitivity high enough to probe the inverted hierarchy of the neutrino mass and even go towards the normal hierarchy region This experiment would be a follow-up to the CUORE experiment with particle identification for background rejection. The same argument can be applied to other physics parameters related to different mechanisms inducing 0ν2β decay, deepening this aspect goes beyond the scope of this paper
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