The process of coherent elastic neutrino-nucleus scattering, CEνNS, is motivated by the very existance of neutral-current neutrino interactions, that allow neutrinos to interact with quarks through the exchange of neutral Z bosons. For neutrino energies below a few tens of MeV, a long-wavelength Z boson can interact with the nucleus as a whole. The occurrence of CEνNS is 2-3 orders of magnitude larger compared to the probability of interacting with the isolated nucleons. Furthermore, compared to other well known neutrino interactions, such as the inverse beta decay, this mechanism does not require neutrinos to have a minimum energy to occur. Nevertheless, CEνNS has evaded any observation for about 40 years since its first theoretical description. From the experimental point of view, indeed, CEνNS can be observed only by measuring the tiny nuclear recoils that it induces. As an example, compared to the aforementioned inverse beta decay, the energy produced by CEνNS is smaller by 4 orders of magnitude.

The first observation of CEνNS was reported by the COHERENT collaboration (2017), which exposed 14.6 kg of sodium-doped CsI scintillating crystals to the neutrino flux produced by a Spallation Neutron Source. Times are now mature to start thinking about high-precision measurements of this process. A measurement performend with an error as low as 1%, indeed, would allow to:

• probe the Standard Model at low energies, for example by measuring with high precision the Weinberg angle at low momentum transfer;
• explore fundamental neutrino properties such as the neutrino magnetic dipole moment;
• search for sterile neutrinos;
• reveal Physics beyond the Standard Model, such as non-standard neutrino interactions;
• support other studies in nuclear physics or supernovae detection;
• offer novel tools for nuclear reactor monitoring.

The increasing interest in a high precision measurement of CEνNS motivated the proposal of the NUCLEUS experiment.

The detectors developed by the NUCLEUS collaboration are cryogenic calorimeters operated at 10 mK. An energy deposit in the cryogenic calorimeter gives rise to a temperature increase that can be converted into an electrical signal using temperature sensors. NUCLEUS will exploit gram-scale crystals based on two well known materials for cryogenic crystals: CaWO₄ and Al₂O₃ (such as the one shown in the next Figure). The temperature sensors for phase-1 will be the Transition Edge Sensors similar to those used by the CRESST experiment in the search for Dark Matter intaractions. The performances of these devices already match the stringent requirements on the energy threshold to measure CEνNS.

To reach an active mass around 10-grams (phase-1) we will deploy two 3x3 arrays of crystals like the one shown in the bottom Figure. We will take advantage from coincidences among the detectors and from the two different materials to better understand our background suorces.

Finally, the detectors will be encapsulated in an outer active detector, which will allow to efficiently reject interactions that are not due to CEνNS, such as muons.

The final setup will be installed in a dilution refigerator at the Very Near Site of the Chooz nuclear power plant (France).

In the first project phase, starting in 2021, we will demonstrate a threshold in the 10 eV range. Such a low threshold will allow to measure for the first time the spectrum of antineutrinos below 1.8 MeV (the threshold of inverse beta decay).

The physics reach of phase-1 will strongly depend on the background, which has never been measured in the energy range of interest and at a shallow overburden. Assuming our benchmark background model, we expect to observe CEνNS (5σ sensitivity) in about 1.5 months of live-time, and a precision of 10 % after one year of measuring time.

To reach the final precision of 1% on the measurement of the CEνNS spectrum, we will need to upgrade the detector to a mass of about 1 kg.

In Italy, we are investigating other technologies for cryogenic calorimeters that would enable to simplify the set-up. Kinetic Inductance Detectors (KIDs) could guarantee a sensitivity similar to the one of the Transition Edge Sensors currently employed by NUCLEUS but with a much simpler read-out. These devices are indeed easily multiplexable, which means that hundreds of KIDs can be measured using a single read-out line and amplifier.

We already proved the possibility of using KIDs in Particle Physics with the CALDER experiment (CALDER Website) and now we are developing a new detector concept to measure CEνNS within the BullKID project.