Result on the Neutrinoless Double Beta Decay Search of ⁸²Se with the CUPID-0 Experiment

Abstract

CUPID-0 is the first large array of scintillating Zn${}^{82}$Se cryogenic calorimeters (bolometers) implementing particle identification for the search of the neutrinoless double beta decay (0$\nu \beta \beta$). The detector consists of 24 enriched Zn${}^{82}$Se bolometers for a total ${}^{82}$Se mass of 5.28 kg and it has been taking data in the underground LNGS (Italy) since March 2017. In this article we show how the dual read-out provides a powerful tool for the $\alpha$ particles rejection. The simultaneous use of the heat and light information allows us to reduce the background down to (3.2${}_{-1.1}^{+1.3}$)×10${}^{-3}$counts/(keV kg year), an unprecedented level for cryogenic calorimeters. In a total exposure of 5.46 kg year Zn${}^{82}$Se we set the most stringent limit on the 0$\nu \beta \beta$ decay ${}^{82}$Se half-life $T_{1/2}^{0\nu }>4.0\times {10}^{24}$ year at 90% C.I.

1. Introduction

The neutrinoless double beta deca $0\nu \beta \beta$ [1] is a transition, in which a nucleus (A, Z) decays into its isobar (A, Z + 2) with the simultaneous emission of two electrons. Both the parent and the daughter nucleus must be more bound than the intermediate one (A, Z + 1) in order to avoid the occurrence of the sequence of two single beta decays. Such a condition, due to the pairing term, is fulfilled in nature for 35 even-even nuclei [2]. This process violates the lepton number by two units; it’s not allowed by the Standard Model of interactions but it’s envisaged in many of its extensions in which neutrinos are their own antiparticles [2]. Its discovery would establish unambiguously the nature of neutrinos as Majorana fermions [3]. In the standard paradigma [2,4] the decay is mediated only by the exchange of three virtual light neutrinos between two charged weak interaction vertices. The chirality mismatch imposed by the V-A structure of the ElectroWeak theory leads to an amplitude proportional to a linear combination of the three neutrino masses. Their squared mass differences are known from neutrino oscillation experiments $\Delta m_{12}^2=m_{{\nu }_2}^2-m_{{\nu }_1}^2\sim 7\times {10}^{-5}$eV${}^2$, $\Delta m_{23}^2=m_{{\nu }_3}^2-m_{{\nu }_2}^2\sim 2\times {10}^{-3}$eV${}^2$ [4,5,6] and their sum is constrained to be less than 0.66 eV at 95% C.L. from cosmological observations but the absolute scale is still unknown [4,5,6]. Three possible orderings are therefore conceivable: normal hierarchy (NH), in which m${}_{\nu 1}$ <m${}_{\nu 2}$ <m${}_{\nu 3}$, inverted hierarchy (IH) where m${}_{\nu 3}$ <m${}_{\nu 1}$ <m${}_{\nu 2}$, and the quasi-degenerate hierarchy (QD), for which masses differences are tiny compared to their absolute values.

At present, no $0\nu \beta \beta$ evidence has been found and actual limits on the decay half-life, in the range of (10${}^{24}$–10${}^{26}$) years [5,6,7,8,9,10,11,12,13], are probing the QD region. The main signature of the $0\nu \beta \beta$ decay is a peak in the sum energy spectrum of the electrons at the Q-value of the reaction which must be resolved over a continuous background [5]. In order to completely explore the IH region ($\tau \approx$10${}^{27-28}$ years), new technologies must be developed, able to reduce the specific background close to zero at the ton × year exposure scale in combination with a sensitive mass of hundreds of kg of isotope and a FWHM energy resolution better than $0.5$% [5,14].

Cryogenic calorimeters (usually called bolometers) play an important role in this field of research [15,16] and in this contribution we review the most recent results obtained by the CUPID-0 experiment.

2. The Detector

A bolometer [17,18] is a single crystal calorimeter operating at ∼10 mK in which the temperature increase, following an energy release inside the crystal itself, is picked-up by a highly sensitive thermometer. This technology allows us to embed the $0\nu \beta \beta$ source in the detector itself and it exhibits excellent energy resolution and very high detection efficiency. One-Ton scale detectors composed of a thousand bolometers could be successfully operated as demonstrated by the CUORE experiment [11,19]. The CUORE sensitivity is limited by the residual background generated by energy-degraded $\alpha$ particles emitted by surface contaminations of the detector material. To suppress $\alpha$ particles the CUPID-0 experiment makes use of scintillating bolometers [20,21,22] in which a small fraction of the released energy is converted into scintillating light. The light escapes the crystal and it is absorbed by a thin bolometer working as light detector. The dual read-out allows us to identify the particle type because $\alpha$ particles have a different light yield and scintillation time-development compared to iso-energetic electrons [23,24,25,26,27].

The CUPID-0 detector is an array of Zn${}^{82}$Se bolometers, enriched in ${}^{82}$Se at ∼95% [28,29,30]. The isotope under study for the $0\nu \beta \beta$ decay is ${}^{82}$Se with a ${\mathrm{Q}}_{\beta \beta }$= 2997.9 ± 0.3 keV [31], well above the energy of the most intense natural high-energy ${}^{208}\mathrm{Tl}$$\gamma$ line at 2615 keV. The $2\nu \beta \beta$ decay of ${}^{82}$Se, allowed by the Standard Model, has a life time ${\tau }_{1/2}$ = (9.39 ± 0.17(stat) ± 0.58(syst)$)\times$10${}^{19}$ year [32] longer enough to reduce to a negligible level the background induced by the pile-up of two $2\nu \beta \beta$ events in the region of interest. The array consists of 24 Zn${}^{82}$Se crystals for a total mass of 9.65 kg, corresponding to 5.13 kg of ${}^{82}$Se and two natural ZnSe crystals (40 g of ${}^{82}$Se mass) not used in the analysis. The number of ${}^{82}$Se nuclei under investigation is (3.41 ± 0.03) × 10${}^{25}$ excluding two crystals featuring poor performances. Each Zn${}^{82}$Se is held in a copper frame by means of small PTFE supports and surrounded by a 3 M Vikuiti plastic reflector to enhance the light collection. The light detector (LD) is a 170-$\mathsf{\mu }$m-thick Ge disk [33] working as a bolometer. One side of the LD is coated with a SiO 60-nm-thick layer to enhance light absorption [34].

Both the LD and Zn${}^{82}$ are equipped with an NTD -Ge thermistor sensor [35], acting as temperature- voltage transducer, and a Si Joule heater [36], which periodically injects a fixed amount of energy to equalise the bolometer gain [37,38].

The signal, amplified and filtered by a six-pole anti-aliasing active Bessel [39,40,41,42,43,44], is recorded by an 18 bit ADC board operating at 1(2) kSPS for the Zn${}^{82}$Se (LD). The detector is anchored to the mixing chamber of an Oxford 1000 ${}^3$He/${}^4$He dilution refrigerator [45] operating at a temperature of about 10 mK and located in Hall A of the Laboratori Nazionali del Gran Sasso (average depth ∼3650 m water equivalent [46]).

The CUPID-0 detector cool down began in January 2017 and, after a period of commissioning it reached stable data taking beginning in May 2017. Here we report the analysis obtained with an a Zn${}^{82}$Se exposure of 5.46 kg year. The results of the first 3.44 kg year data have been published in Ref. [47].

3. Data Analysis and Results

We acquire the complete data stream for both the Zn${}^{82}$Se and the light detector. For the ZnSe, we implement a derivative software trigger while pulses produced by the Joule heater are flagged by the data acquisition system. The waveform is analysed 4 s after the trigger and 1 s before (baseline).

We use the optimum filter algorithm [48,49] to infer the pulse amplitude and the shape parameters. The amplitude is corrected for any shift in thermal gain using the reference pulse injected by the heater every 400 s [37] and the baseline to estimate the detector temperature. We find the amplitude-energy conversion using the most intense $\gamma$ lines produced by a ${}^{232}\mathrm{Th}$ source [47,50].

We discard events on the Zn${}^{82}$Se inconsistent with the filter signal template and events on different bolometers if they occur within 20 ms since they are most likely induced by multiple Compton $\gamma$’s.

The LD signal amplitude and shape is estimated applying the optimum filter at a fixed time delay compared to the Zn${}^{82}$Se heat signal as detailed in Ref. [51]. We build a light shape parameter [27] and we optimize the selection in order to have a unitary efficiency while rejecting the $\alpha$ background and shown in Figure 1. Finally we suppress the background induced by the internal ${}^{208}\mathrm{Tl}$$\beta$/$\beta$ + $\gamma$ decay to ${}^{212}$Pb (${\tau }_{1/2}$ = 3.01 min) tagging the ${}^{212}$Bi (${}^{208}\mathrm{Tl}$ mother ) which $\alpha$ decays with a Q${}_{\alpha }$ = 6207 keV. If the contamination is close to the surface and the $\alpha$ escapes the crystal, only part of the energy of the parent decay is collected. We therefore require the $\alpha$ particle to be in the energy range 2.0–6.5 MeV.

The resulting background index in the region of interest 2800–3200 keV, after the whole selection, results to be BI = (3.2${}_{-1.1}^{+1.3}$) × 10${}^{-3}$counts/(keV kg year).

The total efficiency on the signal $0\nu \beta \beta$ candidate is (75 ± 2)%. It comprises the selection efficiency, evaluated on the most prominent peak in the physics spectrum [52], the ${}^{65}$Zn and the $0\nu \beta \beta$ containment efficiency estimated via GEANT4 simulation. No signal events were found and we set a 90% credibile interval Bayesian lower limit on the ${}^{82}$Se half life $T_{1/2}^{0\nu }>4.0\times {10}^{24}$ year. Details of the analysis and the procedure used to compute the upper limit can be found in Ref. [47,50].

4. Conclusions

CUPID-0 demonstrates that a large array of enriched scintillating bolometers can be successfully operated. The simultaneous readout of the heat and light signals allows us to reject the $\alpha$ induced background and reach the lowest background level ever achieved with bolometric experiments: (3.2${}_{-1.1}^{+1.3}$) × 10${}^{-3}$counts/(keV kg year). This represents a key milestone for the next generation of tonne-scale experiments. In a total exposure of 5.46 kg year Zn${}^{82}$Se we set the most stringent limit on the neutrinoless double beta decay ${}^{82}$Se half-life $T_{1/2}^{0\nu }>4.0\times {10}^{24}$ year at 90% C.I. The data taking is on going and new results will be released in Spring 2019.