Editorial | Published:

Searching the invisible

Nature Physics volume 11, page 881 (2015) | Download Citation

After two Nobel prizes, the quest to uncover new physics continues at the Kamioka site in Japan.

By particle-physics standards, the Kamioka site is a rather small laboratory, hidden away in the mountains of the Gifu prefecture in Japan. Nevertheless, it has come to be seen as something of a legendary place by physicists and science aficionados in Japan. One of its experiments, the Super-Kamiokande, is even reproduced as a 1/10 scale model in Miraikan, the National Museum of Emerging Science and Innovation, in Tokyo. And few Japanese aren't aware of Masatoshi Koshiba, and more recently Takaaki Kajita, the recipients of the 2002 and 2015 Nobel prizes in physics. Although the discovery of neutrinos and their oscillations is one of the more fascinating stories in the history of science, the Kamioka site itself has its own captivating history.

It all started with an old mine dug a kilometre deep below the top of Mount Ikeno, near Kamioka. The stable bedrock and large supply of pure water together with the existing tunnels made the mine an ideal location for a particle physics laboratory. Using 3,000 tonnes of pure water and some 1,000 photomultiplier tubes, the Kamioka Nucleon Decay Experiment, or Kamiokande, was set up to look for signs of proton decay in 1983. Although it initially failed to detect any proton decay signals, Kamiokande observed neutrinos from a supernova in 1987 instead. Then, in 1989, it detected solar neutrinos, and these breakthroughs led to the 2002 Nobel Prize in Physics.

In 1996 Super-Kamiokande (SK) came online — with Kamiokande now standing for Kamioka Neutrino Detection Experiment. With 50,000 tonnes of water and over 11,000 50-cm photomultiplier tubes in the inner detector layer alone, it represented a serious scaling-up of the original Kamiokande detector. The photomultipliers are a wonder in themselves. The largest in the world, they are manually crafted by experts at Hamamatsu Photonics and were first made in 1979 for the Kamiokande. The thinner the glass, the better the signal, but this makes the tubes very fragile and in 2001 an accident compromised many photodetectors. In 2006 the SK resumed operation with all photodetectors covered with protective shields and the experiment has been continuously running ever since. In a decade, only about 100 photomultiplier tubes have malfunctioned, and it will take a while before replacements will be needed.

Every day the SK records about 30 neutrino events, ten of which are atmospheric neutrinos, the rest being solar neutrinos. Their collisions with the nuclei and electrons in the water produce energetic muons and electrons whose Cherenkov light cones leave specific signatures on the photodetector array. This number of neutrino events may seem modest but every day 500 GB of data needs to be analysed and stored.

With some 120 staff and collaborators, the SK is small compared with CERN, but it has been leading neutrino research over the past decades and has been the site where all types of neutrino oscillations have been observed: atmospheric neutrinos in 1998, solar neutrinos (together with the Sudbury Neutrino Observatory in Canada) in 2001, and man-made neutrinos in the K2K and T2K experiments. But these are certainly not the only experiments carried out at the Kamioka Observatory.

Kamiokande was dismantled in 1998 to make space for KamLAND, an ambitious experiment led by Tohoku University. It uses the same detection principles as the Kamiokande and SK, but makes use of liquid scintillator instead of water. It is purified in a special installation to remove the tiniest traces of radioactive material that can lead to false signals. KamLAND has already observed geoneutrinos from the Earth's crust as well as reactor antineutrinos, and it is aiming to test whether neutrinos are Majorana particles by searching for neutrino–antineutrino annihilations from double-beta decay.

Kamioka is conveniently located at roughly the same distance from a number of nuclear plants providing a steady stream of neutrinos and antineutrinos. But following the Fukushima accident in 2011 they were shut down, leaving KamLAND to detect the background. Another unexpected consequence of the Fukushima accident was the contamination of the nylon balloon being assembled in Sendai for the KamLAND-Zen experiment. The experiment uses a 300 kg xenon gas balloon inside the liquid scintillator tank. The level of contamination of the balloon was negligible by any safety standards, but it turned out to have serious impact on the experiment.

Neutrinos are not the only elusive prey being hunted at the Kamioka site. XMASS is searching for dark matter with an 800 kg liquid xenon detector, the world's largest, and a powerful array of photomultipliers. After setting limits for dark-matter detection, XMASS is pushing further. As for SK and KamLAND, however, this requires the elimination of all traces of radioactive contamination even in the photodetectors, and achieving larger detection volumes. Working with Hamamatsu Photonics, the collaboration is developing a new generation of photodetectors. And the upgraded XMASS will also increase the detection volume to 10 tonnes.

The newest experiment at the Kamioka site is under construction: KAGRA will be the first underground interferometer to search for gravitational waves. It required digging some 8 km of new tunnels and its exquisite level of stabilization will require 6 km of vacuum tubes, an ISO class 1 clean room for the laser and sapphire mirrors cryogenically cooled to 20 K. It is expected to become operational in 2017.

With these new experiments on the way, the SK does not plan to rest on its laurels. Its new upgrade aims big: the Hyper-Kamiokande will be twenty times larger than SK, with one million tonnes of water, and 99,000 photodetectors, and is designed to search for signs of CP violation by precisely measuring neutrino and antineutrino oscillations.

Kamioka is certainly not the only place where such ambitious experiments testing fundamental physics are taking place. The search for neutrinos, dark matter and gravitational waves is one of the most exciting frontiers of science, but what is perhaps never sufficiently appreciated and praised is the equally inspiring engineering that makes it possible. This year's Nobel Prize in Physics should be an occasion to celebrate and ponder on the science as well as the scientists and their amazing machines.

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