As stated in the 2020 Update of the European Strategy for Particle Physics, the existence of non-zero neutrino masses is a compelling sign of new physics. The worldwide neutrino physics program explores the full scope of the rich neutrino sector and commands strong support in Europe. Within that program, the Neutrino Platform was established by CERN in response to the 2013 Strategy recommendation and has successfully acted as a hub for European neutrino research at accelerator-based projects outside Europe.

Europe, and CERN through the Neutrino Platform, should continue to support long-baseline experiments in Japan and the United States. In particular, they should continue to collaborate with the United States and other international partners towards the successful implementation of the Long-Baseline Neutrino Facility (LBNF) and the Deep Underground Neutrino Experiment (DUNE).

The Long Baseline Neutrino Facility (LBNF) hosted at Fermilab in Batavia, Illinois, USA, is the facility required for the Deep Underground Neutrino Experiment (DUNE). DUNE is a dual-site experiment for neutrino science and proton decay studies. Once completed, the LBNF will comprise the world's highest-intensity neutrino beam. The infrastructure necessary to support massive, cryogenic far detectors is installed at the Sanford Underground Research Facility (SURF), 1,300 km downstream, in Lead, South Dakota (USA). The experimental facilities, situated 1500 m below the surface, will include several individual cryogenic detectors, each housed inside a large instrumented cryostat filled with 17,500 tons of liquid argon.

The above sketch shows the initial configuration of the DUNE Far detectors.

CERN will, as an in-kind contribution to the LBNF/DUNE project, provide two detector cryostats. CERN has developed a novel technological solution inspired by the liquified-natural-gas (LNG) shipping industry. The patent is based on a membrane-type containment system with two cryogenic liners that support and insulate the liquid cargo. CERN established a fruitful collaboration with Gaztransport & Technigaz (GTT), a firm that deploys LNG in about 80% of all transport ships worldwide. This solution has been successfully implemented in the ProtoDUNE cryostat, serving as a prototype for the cryostat of the DUNE far detector.

The cryostats are freestanding and constructed using membrane cryostat technology. This technology implements a 1.2 mm thick stainless-steel membrane to contain the liquid and transfer the load to the insulation and support structure. The membrane liner is corrugated to provide strain relief resulting from temperature-related expansion and contraction. Each cryostat is passively insulated by a 0.8 m thick layer of polyurethane foam on all sides and the roof. The surrounding steel support structure for the cryostats includes a 12 mm thick stainless-steel plate serving as a vapour barrier, as well as 1.1 m tall I-beams bearing the weight of the cryostat, the enclosed detector, and the contained liquid and gaseous argon.

The design of the supporting structure called the warm structure, has been optimized considering the limited dimensions of the components that can be brought 1500 m down in the assembly caverns at SURF.

The warm structure is composed of 4,400 beam-welded components with a total weight of 4,600 MT and 1,800 plate components with a total weight of 1,180 MT (see above image). The assembly team had to treat meticulously and weld 52,000 M48 sets, 15,000 M16 bolts, 30,000 M14 bolts, and 4,300 m to assemble the warm structure.

Pre-assembly at the factory. Every component is precisely engineered to ensure the highest standards for the DUNE experiment.

Presently, Cryostat #1 warm structure is on its way to the temporary storage site close to SURF. The Cryostat #2 warm structure is being executed according to schedule. It is foreseen to be available for transportation by the end of October 2024. The expected date for the start of the assembly works at SURF is July 2025.

Components ready for packaging.

Watch the first miles of this incredible journey from the heart of CERN to the Sanford Underground Research Facility (SURF) in the US: