Neutrinos are among the most mysterious particles in the Standard Model. Properties such as their mass and their potential role in the universe's matter-antimatter asymmetry remain unknown. Paving the way to a new near detector for Hyper-Kamiokande, the Water Cherenkov Test Experiment (WCTE) aims to shed light on these questions. For the first time, the WCTE collaboration will begin fully operating this pathfinder detector to study how it responds to particles around 1 GeV/c and less at CERN’s East Area T09 test-beamline.

WCTE’s barrel, weighing 40 tonnes when filled with water, stands 3.5 meters high with a 4-meter diameter, designed to replicate the conditions of its potential successor, the Intermediate Water Cherenkov Detector (IWCD), planned for Hyper-Kamiokande in Japan. This setup ensures a similar distance from beam to detector as IWCD’s centre of interaction. Inside, WCTE contains 98 photosensor units, each made of 19 photomultiplier tubes (PMTs), which detect particles based on the pattern of emitted Cherenkov light. These modules, newly developed for the IWCD, represent the next step in detector technology, with the final Hyper-Kamiokande experiment anticipated to contain approximately 20,000 such units. Essential support came from CERN’s BE-EA (Experimental Areas) group, which configured the beamline and adapted the T09 infrastructure to integrate the large-scale detector. To transport and position the detector structure and tank, both with a weight of ~8 ton, the EN-HE group skillfully maneuvered it over the walls of the test-beam area, working with extremely tight crane clearances.

"Launching WCTE marks an exciting leap forward in our understanding of neutrinos and particle detection,” says Mark Hartz (TRIUMF), spokesperson of the WCTE collaboration. “Every piece of data we collect in the East Area's T09 beamline helps refine this technology and sets the stage for future discoveries at Hyper-Kamiokande and other next-generation neutrino experiments."

In addition to calibrating the detector’s response to a few percent precision, WCTE will measure various physics processes to improve simulations of detector behaviour. Following this year's initial test-beam campaign, scheduled to conclude on 27 November, the WCTE team plans to add gadolinium sulphate to the water beginning of 2025 and to continue their tests. Gadolinium enhances the detection of neutron capture events: when a neutron is captured by gadolinium, the nucleus emits a cascade of gamma rays. These gamma rays interact with surrounding water molecules, creating secondary charged particles that produce Cherenkov light, making neutron interactions, such as those from supernovae or neutrinos, easier to identify. “It’s incredibly exciting to see WCTE installed and taking beam in the T09 beamline after years of preparation by both the experiment team and CERN," says Dipanwita Banerjee, BE-EA liaison physicist for the experiment. “The data collected will serve as a crucial input for detector performance in next-generation neutrino experiments.”