Probing new physics with Coherent Elastic Neutrino-Nucleus Scattering and a tabletop experiment

Ever since the Higgs boson was discovered at the LHC in 2012, we had the confirmation that the Standard Model (SM) of particle physics has to be extended. In parallel, the long lasting Dark Matter (DM) problem, supported by a wealth of evidence ranging from precision cosmology to local astrophysical observations, has been suggesting that new particles should exist. Unfortunately, neither the LHC nor the DM dedicated experiments have significantly detected any exotic signals pointing toward a particular new physics extension of the SM so far.

With this research program, we want to take a new path in the quest of new physics searches by providing the first high-precision measurement of the neutral current Coherent Elastic Neutrino-Nucleus Scattering (CENNS). By focusing on the sub-100 eV CENNS induced nuclear recoils, the goal is to reach unprecedented sensitivities to various exotic physics scenarios with major implications from cosmology to particle physics, beyond the reach of existing particle physics experiments. These include for instance the existence of sterile neutrinos and of new mediators, that could be related to the DM problem, and the possibility of Non Standard Interactions that would have tremendous implications on the global neutrino physics program.

To this end, we are building a kg-scale cryogenic neutrino experiment with outstanding sensitivity to low-energy nuclear recoils, called CryoCube, that will be deployed at the ILL nuclear research reactor within the forthcoming Ricochet low-energy neutrino observatory. To provide the required new physics sensitivity, we need to push our detector technology beyond the state-of-the-art performance to reach sub-100 eV energy thresholds with unparalleled background rejection capabilities.

The CENNS research program is structured as follows:
WP1: Single crystal detector design
WP2: From a single crystal to a CryoCube detector array
WP3: Probing new physics with CENNS

During these first two years and a half, the team has focused on:
- Acquiring and setting up the required clean room infrastructure to build our cryogenic detectors in the best conditions. This now allows the team to be more independent and vastly more efficient in our detector design and testing (WP1).
- The Ge technology has significantly progressed, the heat energy resolution using NTD (not NbSi as initially planned) has been achieved on these newly designed 38g bolometers (task 1 - WP1). The electrodes, in two different and both promising designs, have been accomplished and successfully tested and validated (publication upcoming).
- In addition to the electrode design, significant and unplanned efforts have been devoted in the design and validation of a new detector holder achieving a low-capacitance cabling and most importantly an excellent cryostat induced vibrations/friction mitigation. As of summer 2021, the individual Ge-based CryoCube detector design is fully completed and validated (task 2 - WP1).
- New data has been acquired with the Zn technology which has shown some interesting pulses and features but it has been decided to first secure the Ge technology which is much more mature in order to be timely with the future Ricochet experiment installation and guarantee the science outcome from this project from the WP3. Task 3 - WP1 will therefore be delayed to later times.
- We are now moving towards the production phase to build up the whole CryoCube detector array (Task 1 - WP2) considering only Ge-based detectors for now. We are planning on testing a mini-CryoCube detector array by mid-2022.
- A HEMT characterization bench has been developed to design the future HEMT-preamplifier from Task2 - WP2. First noise results are in perfect agreement with our previous models and data (resulting in a publication: 10.1007/s10909-019-02269-5). First HEMT-based preamplifier prototypes are being tested today with some significant delays due to COVID. It should be noted that due to the significant excitement around this development in the cryogenic detector community, other labs (not team members of this ERC) have joined this work in the broader context of the Ricochet experiment that will in the end host the CryoCube detectors (major deliverable of this ERC program combining WP1 and WP2). First charge and heat HEMT-based preamplifier prototypes are being commissioned today with first science results to be expected early-2022.
- In order to fully exploit the scientific outcome of the data from the future Ricochet experiment at ILL, a new processing pipeline has been developed and will allow us, together with a joint detector calibration program, to control the systematic uncertainties of our CENNS measurement with the CryoCube to the percentage level precision (upcoming publication)
- A significant amount of work has been done to secure the construction of the future Ricochet experiment by finding additional partners (10 institutions between USA, France and Russia), funding and resources, and a nuclear reactor site (ILL). The PI of this ERC-CENNS program has now become the spokesperson of this future experiment, based on an international collaboration of over 80 physicists, engineers and technicians. Ricochet is now secured and is planned for integration at ILL by 2022-2023. Though not directly related to this ERC program these progresses are pivotal to achieve the final science outcome of this ERC-CENNS research program by providing the required experimental infrastructure to host the CryoCube and deliver the first low-energy and high-precision measurement of the coherent neutrino process (WP3). Therefore, this parallel effort was absolutely essential to guarantee the full success of this ERC research program by ensuring the feasibility of the final scientific outcome.

Based on our ongoing progress and research program, the next steps are results are:
- Achieving world leading ionization resolution thanks to first HEMT-base preamplifier prototype combined with our low-capacitance detector design
- Scale up the electronics to readout the whole CryoCube detector array
- Validation of the our new Ge detector design before scaling up their numbers to deliver by 2022 the CryoCube detector array
- Fully commission the CryoCube with its electronics in the low-vibration and low-background Ricochet cryostat
- Deploy the Ricochet cryostat with the CryoCube at ILL to deliver the first low-energy and high precision CENNS measurement to search for new physics in the electroweak sector

Periodic Reporting for period 2 - CENNS (Probing new physics with Coherent Elastic Neutrino-Nucleus Scattering and a tabletop experiment)

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