Nestled within the confines of CERN’s Neutrino Platform on the Prévessin campus in France are two colossal structures encased in a vibrant red grating. These structures, chambers resplendent in gleaming stainless steel, form the cryostat modules of the ProtoDUNE experiment. Although sizable in their own right, they pale when juxtaposed against their successors, the behemoths being constructed for the Deep Underground Neutrino Experiment (DUNE) in the USA. The Neutrino Platform is also the proud home to an assembly station for the Tokai to Kamioka (T2K) experiment, a monumental neutrino facility nestled in the heart of Japan.
The elusive neutrinos represent one of the least understood constituents of the Standard Model. Despite their status as the most populous massive particles in the cosmos, neutrinos carry minuscule mass and interact exclusively through gravity and the weak nuclear force, rendering them challenging subjects for study. However, these seemingly inconsequential particles might hold the keys to pivotal enigmas, such as the reason behind the Universe’s matter-antimatter asymmetry. Long-baseline neutrino-oscillation experiments could potentially resolve these conundrums by investigating how neutrinos alter their “flavour,” or oscillate, as they traverse substantial distances, or baselines.
Upon completion in the USA, DUNE will project a neutrino beam from the Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois, a remarkable journey of over 1300 kilometres through the Earth to neutrino detectors nestled 1.5 km beneath the surface at the Sanford Underground Research Facility (SURF) in Sanford, South Dakota. These detectors, colossal cryostats filled with liquid argon, are designed to detect the rare instances when neutrinos interact with the argon, ionising its atoms. An electric field running through the detector segregates the dislodged electrons and argon atoms. The resulting electron cloud, bearing the imprint of the ionisation, is detected by electrode sensors located on the cryostat walls. This process renders vivid images of the trajectories of particles birthed by neutrino interactions, enabling physicists to decipher the neutrinos’ properties such as their flavour and mass. These detectors, exploiting a blend of electric fields coursing through a fluid volume, are known as time projection chambers.
Flashback to Prévessin. In 2018, ProtoDUNE embarked on its inaugural run. Both cryostats were rigorously tested until 2021, the first employing a single-phase configuration of the experiment (ProtoDUNE-SP) and the second utilizing a dual-phase setup (ProtoDUNE-DP). The first run documented over four million particle interactions, shedding invaluable insights on the technological hurdles associated with DUNE, and demonstrated that the full-scale experiment was ready for construction. As of January 2023, the Neutrino Platform has been bustling with preparations for ProtoDUNE’s second run. Both cryostats now operate on a single-phase, with one analyzing the drift of electrons across a horizontal electric field (ProtoDUNE-HD) and the other studying a vertical field (ProtoDUNE-VD). This second run will help determine the optimal implementation of these technologies in DUNE. The cryostats will be soon filled with liquid argon and commence data acquisition at the dawn of the coming year.
The Neutrino Platform also serves as the assembly ground for the T2K experiment. Having been operational for more than ten years in Japan, T2K propels neutrinos from the eastern coastline at Tokai, covering a substantial distance of 295 km to the Super-Kamiokande detector situated in Kamioka, near the western coast. In the year 2011, T2K offered the inaugural evidence of muon neutrino transforming into electron neutrino oscillations and has since hinted at neutrino asymmetry between matter and antimatter. Currently, an upgrade is underway for one of its detectors, ND280, a process that the T2K collaboration expects will amplify the experiment’s efficiency and provide a more accurate reconstruction of neutrino oscillations.
The comprehensive overhaul of ND280 involves multiple subdetectors, the majority of which were assembled and trialed at the Neutrino Platform. This includes the incorporation of fresh time projection chambers, one of which is currently tasked with capturing cosmic data at CERN. Other varieties of subdetectors have either been successfully installed or are on standby for transportation to Japan, post-assembly at the Neutrino Platform. In addition to the individual subdetectors, the gas system for the ND280 detector as a whole was constructed and meticulously examined at CERN. The concluding steps involve assembling an additional time projection chamber, its shipment to T2K, followed by its installation. The makeover of ND280 is anticipated to be finalized in 2023. Once upgraded, ND280 is poised to play a significant role in the upcoming generation of long-baseline neutrino oscillation experiments, colloquially termed as Hyper-Kamiokande (HyperK).