The giant under the mountain

But let’s begin at the beginning. What is Hyper-Kamiokande and why are neutrinos so popular in particle physics? Neutrinos are such a hot topic because scientists expect them to reveal facts about the universe that have puzzled them for a long time. Why does matter dominate over antimatter? Is there something we’re overlooking in the Standard Model of particle physics? Unfortunately, neutrinos like playing hard to get. They hardly interact at all with anything, which makes catching and studying them very complicated. Several neutrino experiments have been and are currently running; more are being planned and built all over the world. Some are embedded into the ice at the South Pole, some look for neutrinos from the Sun in disused mines, receive beams from hundreds of kilometres away or study the elusive particles around cores of nuclear reactors. (By the way, if you want to know more about them and all the ways Swiss institutes are involved in neutrino research check this link).

One of the experiments that is buried deep underground to block out distracting cosmic rays is Hyper-Kamiokande. It is under construction in Gifu Prefecture in central Japan, a region known for its mountain ranges and zinc, lead and silver mines. One of these mines has been host to neutrino experiments for a long time, including the one that in 1998 found that neutrinos can transform from one kind into another spontaneously, proving that they have mass and winning it the Nobel Prize in Physics in 2015. Hyper-Kamiokande is going to be the newest and by far the biggest installment in this series of huge underground neutrino observatories. When it comes online in a few years’ time, it will be twenty times more powerful than its predecessor, Super-Kamiokande. Last month, excavation work was completed for the massive underground cavern that is up to 69 metres wide and 73 metres high, sitting 600 metres below the surface. Over the coming years it will be equipped with 20’000 photomultiplier tubes (PMTs), a technology already used and well understood in the current experiment. PMTs are highly sensitive light detectors that capture faint flashes of light produced when neutrinos interact with matter. The matter in question is going to be ultra-pure water: the cavern will be flooded with an amount of pure water equivalent to a hundred Olympic-size swimming pools. When a neutrino interaction in the water produces charged particles, these emit Cherenkov radiation that is then detected by arrays of PMTs lining the tank walls.

These arrays are firmly in the grip of researchers from Europe, with Swiss institutes playing a key role. They have come up with a solution to connect PMTs with each other and make the setup more self-sufficient than its predecessors. “Hyper-Kamiokande will be too big to connect all PMTs by cable,” explains André Rubbia, professor at ETH Zurich. “We have come up with a solution that connects 24 PMTs with each other in one underwater vessel that contains all the electronics needed to read out and control the phototubes as well as a high-voltage power supply.”

A total of 950 of these vessels will be needed to cover the surface area. They will be assembled and tested at CERN’s Neutrino platform in a European teamwork effort overseen by Davide Sgalaberna, professor at ETH Zurich. The team is at the end of the procurement phase, getting the remaining power supplies to CERN by December and all other components early next year. The next steps are to set up an assembly chain for the vessels, calibrate the electronics and run several tests, including vibration tests, a test-run in a high-pressure water tank, and one-year long stability tests. “We expect to be quite busy over the next years,” Rubbia says.

However, the brand new Hyper-Kamiokande detector isn’t the only thing that’s under Swiss control. The 600-people, 22-country collaboration has come up with an ingenious schedule to make sure optimal results are guaranteed at all times – and to keep everybody on their toes. That’s because the enormous underground cavern is just one place where the physics is done – a lot of physics also happens near the research centre J-PARC 250 kilometres further to the east, where the beam of neutrinos is sent on its way and checked by the “near detector” before the neutrinos can transform. Because only when you know what goes in (seen by the near detector) you will be able to check against what comes out (over in the new Hyper-K “far detector”). Part of the near detector has just undergone a major upgrade (see here), serving the running T2K experiment, and will be in routine operation when Hyper-K starts up. The Swiss researchers are now developing the conceptual design of the possible ultimate upgrade of the near detector, so that there’s always work going around for them and their students.

“This is one of the great advantages of being involved in a working experiment and a not very large collaboration,” says Federico Sanchez Nieto, professor at the University of Geneva, former spokesperson of the T2K collaboration and key contributor to the near detector’s recent upgrade. “As a student you get to work on all stages of the experiment: the design, the assembly, the testing, data taking and analysis. It’s a great education!” Rubbia concludes: “We’re feeling good and fortunate to make this contribution. I expect a major breakthrough in understanding CP violation to come out of Hyper-Kamiokande.” We’ll keep you updated on the electronics assembly and testing and other project milestones here.

Barbara Warmbein