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Register a new userDiscovery probability of next-generation neutrinoless double-$beta$ decay experiments
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The Bayesian discovery probability of future experiments searching for neutrinoless double-$beta$ decay is evaluated under the popular assumption that neutrinos are their own antiparticles. A Bayesian global fit is performed to construct a probability distribution for the effective Majorana mass, the observable of interest for these experiments. This probability distribution is then combined with the sensitivity of each experiment derived from a heuristic counting analysis. The discovery probability is found to be higher than previously considered, but strongly depends on whether the neutrino mass ordering is normal or inverted. For the inverted ordering, next-generation experiments are likely to observe a signal already during their first operational stages. Even for the normal ordering, in the absence of neutrino mass mechanisms that drive the lightest state or the effective Majorana mass to zero, the probability of discovering neutrinoless double-$beta$ decay can reach $sim$50% or more in the most promising experiments.
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In the last two decades the search for neutrinoless double beta decay has evolved into one of the highest priorities for understanding neutrinos and the origin of mass. The main reason for this paradigm shift has been the discovery of neutrino oscillations, which clearly established the existence of massive neutrinos. An additional motivation for conducting such searches comes from the existence of an unconfirmed, but not refuted, claim of evidence for neutrinoless double decay in $^{76}text{Ge}$. As a consequence, a new generation of experiments, employing different detection techniques and $betabeta$ isotopes, is being actively promoted by experimental groups across the world. In addition, nuclear theorists are making remarkable progress in the calculation of the neutrinoless double beta decay nuclear matrix elements, thus eliminating a substantial part of the theoretical uncertainties affecting the particle physics interpretation of this process. In this report, we review the main aspects of the double beta decay process and some of the most relevant experiments. The picture that emerges is one where searching for neutrinoless double beta decay is recognized to have both far-reaching theoretical implications and promising prospects for experimental observation in the near future.
We comment on the recent claim for the experimental observation of neutrinoless double-beta decay. We discuss several limitations in the analysis provided in that paper and conclude that there is no basis for the presented claim.
NEXT-100 is an electroluminescent high-pressure xenon gas time projection chamber that will search for the neutrinoless double beta ($beta beta 0 u$) decay of Xe-136. The detector possesses two features of great value for $beta beta 0 u$ searches: energy resolution better than 1% FWHM at the $Q$ value of Xe-136 and track reconstruction for the discrimination of signal and background events. This combination results in excellent sensitivity, as discussed in this paper. Material-screening measurements and a detailed Monte Carlo detector simulation predict a background rate for NEXT-100 of at most $4times10^{-4}$ counts keV$^{-1}$ kg$^{-1}$ yr$^{-1}$. Accordingly, the detector will reach a sensitivity to the bbonu-decay half-life of $2.8times10^{25}$ years (90% CL) for an exposure of 100 $mathrm{kg}cdotmathrm{year}$, or $6.0times10^{25}$ years after a run of 3 effective years.
We discuss a mechanism of neutrinoless double beta decay, where neutrinos of different flavours come into play. This is realized by effective flavour-violating scalar interactions. As one consequence, we find that within the normal mass ordering the neutrino effective mass may no longer vanish due to contributions from other flavours. We evaluate the necessary nuclear matrix elements, consider the interference between the standard diagram and the new scalar one, and analyze a UV-complete model that realizes the scalar interaction. Tests of the complete model are possible at colliders and future neutrino experiments. Our scenario represents an alternative mechanism for neutrinoless double beta decay, where nevertheless lepton number violation resides only in Majorana mass terms of light neutrinos.
Tremendous efforts are required to scale the summit of observing neutrinoless double beta decay ($0 u beta beta$). This article quantitatively explores the interplay between exposure (target mass X data taking time) and background levels in $0 u beta beta$ experiments. In particular, background reduction can substantially alleviate the necessity of unrealistic large exposure as the normal mass hierarchy (NH) is probed. The non-degenerate (ND)-NH can be covered with an exposure of O(100) ton-year, which is only an order of magnitude larger than those planned for next generation projects - provided that the background could be reduced by 0($10^{-6}$) relative to the current best levels. It follows that background suppression will be playing increasingly important and investment-effective, if not determining, roles in future $0 u beta beta$ experiments with sensitivity goals of approaching and covering ND-NH.
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