Experimental objectives and impact
The impact of BINGO in the search for 0n2b will be at three levels: (i) validation of ground-breaking technologies for the control of the background in MINI-BINGO; (ii) application of these technologies to CUPID, the proposed follow-up to CUORE experiment in the Gran Sasso underground laboratory (LNGS), increasing substantially its currently estimated sensitivity; (iii) pave the way to large long-term 0n2b experiments able to demonstrate the Majorana nature of neutrinos even in case of vanishing lightest neutrino mass and direct ordering of the three neutrino masses.
Composition of MINI-BINGO
OBJECTIVE 1 – Develop a low-background technology demonstrator dubbed MINI-BINGO
MINI-BINGO will be an array of scintillating bolometers and of scintillators operated at ~15 mK in a dedicated dilution refrigerator located in the Modane underground laboratory (LSM) in France or in LNGS in Italy. The three sections of the demonstrator will consist of TeO2 Cherenkov bolometers, Li2MoO4 scintillating bolometers and of ZnWO4 scintillators. The first two sections consist of the two compounds selected for the BINGO technology, aiming at the study of the isotopes 130Te and 100Mo. The crystals will be grown with materials of natural isotopic composition, as the goal of MINI-BINGO is to validate a low-background technology and not to perform a 0n2b experiment. I stress that protocols to develop enriched crystals of 130TeO2 and 100Li2MoO4 are already available [30,31] and are not relevant in the BINGO program. The third section will work as an active almost-hermetic internal background shield, operated at the same temperature as the 0n2b-material sections and in close proximity to them. I remark that it will be the first time that a large array of bolometers will be surrounded by an active shield directly in the dilution-refrigerator experimental volume. Some characteristics of the three sections are summarized in Table 2. A cross section of the setup is shown in Fig. 4.
The quantitative statement related to Objective 1 is the demonstration that a background index of the order of b ~ 10‑4 ckky is achievable with the BINGO technology. This can be proved in MINI-BINGO considering that the total mass of the 0n2b materials is 13.6 kg. If no count is observed in 1 y of data taking in the energy range 2.5-3.5 MeV, containing both 130Te and 100Mo Q-values, the limit at the 90% c.l. set on the background index will be: b < 2.44 / (1000×13.6×1) ckky = 1.8×10-4 ckky.
OBJECTIVE 2 – Develop innovative bolometric light detectors with a-few-optical-photon sensitivity
The central element of the BINGO methods for background reduction is an innovative bolometric light detector based on the Neganov-Luke effect. Prototypes (on which I have extensively worked) have shown that the rms baseline width that can be reached with these devices is of the order of 10 eV. A further reduction is in principle possible (by at least a factor 5); in addition, a protocol for a series production of these detectors, with reproducible performances, must be set up. These are two goals of BINGO. The good performance of the light detector is required for several reasons:
- Detection of Cherenkov light in TeO2, with crystals in an open structure without reflectors
- Rejection of the background induced by random coincidences of 2n2b events in Li2MoO4 (relevant for the enriched crystals) through pile-up identification in the light channel
- Detection with low threshold (< 50 keV) of the scintillation light in ZnWO4 rods long up to ~40 cm
- Rejection of the surface radioactivity through minimization of passive surfaces facing the crystals
OBJECTIVE 3 – Proposal to implement the BINGO methods in CUPID or in an equivalent setup
It is essential to verify the success of the BINGO technology by proving a background index of the order of b ~ 10‑5 ckky. This cannot be achieved in a small-scale bolometric setup as that realizable in the framework of an ERC project, and even the time scale is not adequate. The purpose of BINGO is to install its ground-breaking solutions in the CUPID experiment, either at the very beginning or in a second phase of the data taking, depending on the timeline of CUPID (currently unknown). The CUPID collaboration has in any case chosen to perform the experiment in a phased approach, installing initially about 1/3 of the total sensitive mass and the remaining 2/3 after about 3 years. This would be compatible with an implementation of the BINGO approach in the second phase of the experiment even if CUPID started the data taking as early as 2022, which is quite unrealistic. It is to note that I am one of the nine members of the CUPID steering committee and technical coordinator of CUPID-Mo (the most important CUPID demonstrator running in LSM in cohabitation with the bolometric dark-matter experiment EDELWEISS). I will have therefore a lot of opportunities to describe and discuss the progress of the BINGO solutions inside the CUPID collaboration.
The quantitative statement related to Objective 3 is the demonstration that a background index of the order of b ~ 10‑5 ckky is achievable with the BINGO technology, both for 100Mo and 130Te. This can be proved by running CUPID in the BINGO mode (I will dub this experiment CUPID/BINGO). To fix the ideas, I propose that 240 kg of TeO2 and 330 kg of Li2MoO4 be installed in the current CUORE cryostat (Fig. 3). In condition of zero background, the level b ~ 10‑5 ckky will be demonstrated in about 7 y (10 y) running time in an energy window of 100 keV around the Q-value of 100Mo (130Te). The 90% c.l. sensitivity to mbb of this experiment will be 7.7-13.8 meV for 100Mo, 10.1-15.7 meV for 130Te and 7.1-12.3 meV in combined mode. This corresponds to an extremely sensitive next-generation experiment capable to explore the inverted hierarchy region with two isotopes simultaneously.
A complementary option is to adopt the BINGO methods in a CUPID-like experiment to be performed in China, in the CJPL underground laboratory. Let me remark that there is a large CUPID community in China, partially connected with the CUORE experiment, which intends to perform a next-generation 0n2b bolometric experiment on Chinese soil in parallel with CUPID in LNGS .