Sense and sensitivity of double beta decay experiments

Abstract

The search for neutrinoless double beta decay is a very active field in which the number of proposals for next-generation experiments has proliferated. In this paper we attempt to address both the sense and the sensitivity of such proposals. Sensitivity comes first, by means of proposing a simple and unambiguous statistical recipe to derive the sensitivity to a putative Majorana neutrino mass, m ββ . In order to make sense of how the different experimental approaches compare, we apply this recipe to a selection of proposals, comparing the resulting sensitivities. We also propose a "physics-motivated range" (PMR) of the nuclear matrix elements as a unifying criterium between the different nuclear models. The expected performance of the proposals is parametrized in terms of only four numbers: energy resolution, background rate (per unit time, isotope mass and energy), detection efficiency, and ββ isotope mass. For each proposal, both a reference and an optimistic scenario for the experimental performance are studied. In the reference scenario we find that all the proposals will be able to partially explore the degenerate spectrum, without fully covering it, although four of them (KamLAND-Zen, CUORE, NEXT and EXO) will approach the 50 meV boundary. In the optimistic scenario, we find that CUORE and the xenon-based proposals (KamLAND-Zen, EXO and NEXT) will explore a significant fraction of the inverse hierarchy, with NEXT covering it almost fully. For the long term future, we argue that 136 Xe-based experiments may provide the best case for a 1-ton scale experiment, given the potentially very low backgrounds achievable and the expected scalability to large isotope masses.

Nuclear matrix elements for double-βdecay

“…This multiplicative factor was taken into account in Ref. [50] when comparing IBM-2 matrix elements with the ISM. The discrepancy between IBM-2 and QRPA-Tü multiplicative factors is not understood and should be investigated further.…”

The NEXT experiment: A high pressure xenon gas TPC for neutrinoless double beta decay searches

“…For a total exposure of 500 kg · year, the sensitivity is better than 100 meV. This sensitivity can match or outperform that of the best experiments in the field [4].…”

“…However, due to the finite energy resolution of any detector, ββ0ν events are reconstructed within a non-zero energy range centered around Q ββ , typically following a gaussian distribution. Other processes occurring in the detector can fall in that region of energies, thus becoming a background and compromising drastically the experiment's expected sensitivity to m ββ [4].…”

Radiative neutrino mass generation linked to neutrino mixing and0νββ-decay predictions

“…Note, however, The nuclear matrix elements (NMEs) |M ν | are the averaged best estimates from Ref. [77]. The NMEs M SD are the values given in Ref.…”

“…Equation (5.22) then allows us to derive limits on m ν ee or ǫ 3 from experimental bounds on T 0νββ 1/2 . The values of G 01 , M ν and M SD are taken from [76], [77] and [78], respectively, and are presented in Table 1. Note, however, The nuclear matrix elements (NMEs) |M ν | are the averaged best estimates from Ref.…”