The LiquidO detection technology and technique were conceived in late 2012 by Dr. A. Cabrera, scientist from the CNRS/IN2P3 and affiliated then to the Astroparticle and Cosmology Laboratory (APC, Paris) – now at the Irene Juliot-Curie Laboratory (IJCLab, Orsay) – in the context of new scientific developments for the Double Chooz reactor neutrino experiment in fundamental particle physics research. The original goal, at the time, was an attempt to achieve antimatter positron (e+; signal) identification to reduce the large cosmogenic backgrounds (predominantly β– particles) of the experiment due to its shallow overburden configuration at the LNCA underground laboratory, located at Chooz. The studies demonstrated that the positron identification was indeed impractical; so far effectively impossible in the context of transparent detectors such as Double Chooz. Instead, the studies implied the birth of LiquidO, as an accidental outcome. The subsequent effort and first experimental developments of LiquidO were since led by A. Cabrera along with (alphabetically ordered): Dr. H. de Kerret (CNRS/IN2P3, APC, Paris), Dr. M. Obolensky (CNRS/IN2P3, APC, Paris), Prof. F. Yermia (CNRS/IN2P3, Subatech, Nantes, France) in France. The concept was later on confidentially discussed with (time-ordered) Prof. F. Suekane (Tohoku University, RCNS, Sendai, Japan) and Dr. C. Buck (Max-Plank Institute, Heidelberg, Germany). All those scientists participated, in different ways, in the initial shaping of LiquidO in the period 2012-2016.
Until 2016, all LiquidO developments were kept confidential while two European Research Council (ERC) consolidating grants proposals were attempted for funding the initial R&D. The first ERC was submitted in 2014 (classed “A” upon evaluation) but unfunded in the final evaluation stage. A similar outcome happened in 2015 (second submission), thus largely frustrating – and even truncating – the hope for a small group stand-alone R&D strategy for LiquidO due to lack of funding and limited dedicated personnel, as all the above-mentioned scientists were heavily involved in leading the Double Chooz physics programme. Hence, in 2016, the LiquidO scientific collaboration/consortium was founded by A. Cabrera for the R&D development of the technology as well as the first studies of the possible derived physics programmes. The first R&D support was granted via the success of the Blaise Pascal Chair awarded to Prof. F.Suekane by the École Normal Superieur (Paris, France), thus becoming a visiting professor at the APC between 2016-2018 in the context of CNRS/IN2P3 with A. Cabrera as host. This fellowship provided the first dedicated resources, including a researcher (Dr. S. Wagner) to work fully in LiquidO R&D. At almost the same time, an EU-Marie Skłodowska-Curie fellow (Dr. M. Grassi) also started under the supervision of A. Cabrera. The reinforced team enabled the first full simulation-based studies consolidating LiquidO’s first conceptual proof of principle. In 2017, the first LiquidO CNRS/IN2P3 national collaboration was formed, by A. Cabrera, out of 6 national laboratories (APC, CENBG, CPPM, LAL, LNCA, Subatech) enabling extra support, including some minor funding, for a coherent experimental demonstration with prototype strategy in France. The programme was realised in the context and with the support of the LiquidO international consortium, with about 20 institutions from over 10 countries; mainly Europe along with the Americas and Asia. In the 2016-2018 period, the main focus remained on the prospective technological applications to fundamental neutrino physics research along with some initial exploration of possible ββ detection. In parallel, some preliminary research efforts targeted LiquidO’s application to both collider physics calorimetry and the detection of radioactive gases.
In 2018, the first-ever public presentation of LiquidO, at a scientific international major conference, was done during the Neutrino Oscillation Workshop 2018 (NOW-2018, Ostuni, Italy) entitled “Physics Potential of the Novel LiquidO Technique” in mid-September 2018. This release benefited from the just obtained first experimental proof-of-principle using the data of the Micro-LiquidO prototype detector, led by the LiquidO consortium with major contributions by the CNRS/IN2P3 team. This release was preceded by a preliminary presentation during the Neutrino-2018 conference (Heidelberg, Germany) in June 2018, where the conceptual LiquidO potential was briefly highlighted to the community. However, the first official full public release of the technology awaited until the 7th June 2019 in a dedicated CERN Detector Seminar, delivered by A. Cabrera on behalf of the LiquidO Consortium and entitled “LiquidO: Novel Opaque Neutrino Detection Technology“. The seminar timing was orchestrated with the completion of the first publication write-up that was in preparation for an imminent release and followed immediately by the presentation at one of the most important international conferences in the field, the TAUP-2019 (Toyama, Japan) entitled “LiquidO: A Novel Neutrino Detection Technique” by F. Suekane.
A major milestone was the first LiquidO publication (A. Cabrera et al), released as arXiv:1908.02859 entitled “Neutrino Physics with an Opaque Detector” on the 7th of August 2019. This publication covered the technique concept and demonstrated with both simulation and our first prototype data along with some discussion of the neutrino physics potential for reactor, solar, and supernova neutrino detection as well as geoneutrino. This release was orchestrated to be followed by the complementary publication release (C. Buck et al) as arXiv:1908.03334 entitled “Novel Opaque Scintillator for Neutrino Detection” on the 9th of August 2019 as motivated by LiquidO. This latter publication covered the first opaque scintillator conceived within the LiquidO consortium, which has been used heavily in LiquidO’s prototyping and demonstration. The edition of the LiquidO publication was led (alphabetically) by A. Cabrera, Prof. J. Hartnell (Sussex University, UK) and Prof. P. Ochoa-Ricoux (University of California, US). The manuscript was submitted to Nature Portafolio, proposing its Communication Physics journal for submission. The publication (Communication Physics 4 273) was accomplished on the 21st of December 2021, after some long COVID-related delays during the reviewing.
The second experimental data release of LiquidO was done during the Neutrino-2022 conference on the 5th of June 2022 in a dedicated talk entitled “LiquidO Opaque Neutrino Detection: New Results & Status“. The first data results from the MINI(e)-LiquidO prototype detector were shown to successfully demonstrate the expected and now extended LiquidO performance in terms of opacity, event topology, calorimetry and the preliminary light level potential in both transparent and opaque modes in both liquid and solid (crystalised opaque scintillator) phases. The prototype’s data also exhibited the potential of excellent timing enabling possible discrimination between Cherenkov and scintillator light as a possible detection handle in some cases. These results are expected to be published soon and they embody the main to-date demonstration of LiquidO; a TRL level of 5-6.
Other prototype systems are in preparation to further experimental demonstration, including detailed studies of particle identification and full engineering solutions. Both are geared to support and optimise the design of the forthcoming small multi-ton detectors (approved) for LiquidO-based projects. Together a few of our projects are funded (alphabetically) by the Agence National de la Recherche (ANR, France), the European Innovation Council (EIC of the EU), and the UK Research and Innovation (UKRI, UK) at the level of several millions of euros. The core R&D however still relies on some limited national funds, mainly from the CNRS/IN2P3 (France) and dedicated resources from our consortium institutions, such as the IJCLab (Orsay), etc.