Final Result of CUPID-0 Phase-I in the Search for the Se82 Neutrinoless Double- β Decay

Abstract

CUPID-0 is the first pilot experiment of CUPID, a next-generation project for the measurement of neutrinoless double beta decay (0νDBD) with scintillating bolometers. The detector, consisting of 24 enriched and 2 natural ZnSe crystals, has been taking data at Laboratori Nazionali del Gran Sasso from June 2017 to December 2018, collecting a 82 Se exposure of 5.29 kg×yr. In this paper we present the phase-I results in the search for 0νDBD. We demonstrate that the technology implemented by CUPID-0 allows us to reach the lowest background for calorimetric experiments: (3.5 +1.0 −0.9 ) × 10 −3 counts/(keV kg yr). Monitoring 3.88×10 25 82 Se nuclei×yr we reach a 90% credible interval median sensitivity of T 0ν 1/2 > 5.0 × 10 24 yr and set the most stringent limit on the half-life of 82 Se 0νDBD: T 0ν 1/2 > 3.5 × 10 24 yr (90% credible interval), corresponding to m ββ < (311-638) meV depending on the nuclear matrix element calculations.Nowadays, neutrinoless double beta decay (0νDBD) is considered one of the most sensitive probes for Physics Beyond the Standard Model. This hypothetical nuclear transition foresees the simultaneous decay of two neutrons into protons and electrons without the emission of neutrinos [1]. Its detection would prove the nonconservation of the total lepton number, setting an important milestone for leptogenesis and baryogenesis theories [2]. The observation of 0νDBD would give precious insights also in Particle Physics, as this transition can occur only if neutrinos coincide with their own antiparticles, according to the Majorana hypothesis. As a consequence, its detection would allow to establish the fundamental nature of these particles [3]. Furthermore, if the mechanism at the basis of 0νDBD is the exchange of light Majorana neutrinos, the half-life of the transition (T 0ν 1/2 ) would scale as T 0ν 1/2 ∝ m −2 ββ , where the parameter m ββ (effective Majorana neutrino mass) is a superimposition of the neutrino mass eigenvalues m i weighted by the elements of the neutrino mixing matrix (U ei ): m ββ =| i U 2 ei m i | [4]. Thus, a measurement of T 0ν 1/2 would also allow to constrain the absolute mass scale of neutrinos.The increasing interest in the search for 0νDBD motivated a huge international effort in the development of several technologies [5-9] to study some of the candidate isotopes: elements with even atomic number and even neutron number, for which the single beta decay is strongly forbidden by energy conservation law. These experiments are now envisioning next-generation detectors to probe half-lives exceeding 10 27 yr and, thus, the whole range of the inverted hierarchy region of neutrino masses, whose lower bound is at m ββ ∼10 meV. The main handles to improve the sensitivity are the detector exposure, the background in the region of interest and the energy resolution [10]. To ensure a competitive discovery potential, next-generation detectors will need more than 10 27 emitters (hundreds of kg of source), a background as close as possible to zero in 5-10 years of data-taking, and a...

The CUPID-Mo experiment for neutrinoless double-beta decay: performance and prospects

“…We expect to achieve a 5 keV energy resolution (FWHM) at Q β β with a dedicated optimization of the energy reconstruction algorithms. With an average containment efficiency for 0νβ β decay events of 75%, and assuming a ∼ 90% analysis efficiency, as obtained in CUORE [14] and CUPID-0 [27] for the combined trigger efficiency, multiplicity, pulse shape analysis (PSA) and RLY cut efficiencies we obtain the exclusion sensitivity curves reported in Fig. 18.…”

“…With an exposure of ∼10 kg×yr, CUPID-0 proved that dual readout could suppress the dominant α background to a negligible level, obtaining the lowest background level for a bolometric experiment to date [26]. Nevertheless, the radiopurity and energy resolution (20.05±0.34 keV FWHM at Q β β ) of the CUPID-0 crystals [26,27] do not meet the requirements of CUPID and demand further R&D activity if Zn 82 Se were to be used.…”

First search for Lorentz violation in double beta decay with scintillating calorimeters

“…To evaluate the effects of this possible contamination, a test has been performed including 90 Sr in the list of sources. As reported in [15], the energy calibration is affected by bias evaluated over all the interest energies. To control the effects of this bias on the current analysis, an evaluation of the obtained limit is performed using the corrected energy scale.…”

“…The dual heat/light readout allows us to combine the excellent energy resolution of bolometers with the background rejection capabilities of scintillators. Indeed, CUPID-0 reached the lowest background ever measured in a 0νββ bolometric detector, setting the most stringent limits in the search of the 82 Se 0νββ to the fundamental and excited states of 82 Kr [14][15][16]. A comprehensive background model has also been developed for CUPID-0 [17], evaluating and localizing all the possible sources of background for the detector.…”

Evidence of Single State Dominance in the Two-Neutrino Double- β Decay of Se82 with CUPID-0

“…The uncertainty on the energy scale is investigated exploiting a dedicated calibration with a 56 Co source. The distribution of residuals shows a parabolic dependence on energy, with a maximum deviation from zero of 3 keV between 511 keV and the 2νββ -decay endpoint at 3 MeV [31]. We take into account this effect by adding to the nominal energy of each event the corresponding residual, evaluated from the distribution.…”