Visiting the Japanese Super-Kamiokande detector (part 2)
Big questions around small neutrinos
In deep underground tunnels of former mines near the Japanese Alps, teams of scientists with Swiss participation are researching various types of elementary particles. Over the next few years, powerful research instruments will be put into operation with which scientists want to discover the nature of neutrinos. The hoped-for results could lead to solving of deep puzzles in our understanding of the universe.
Throughout the world there are mountains perforated by long tunnel systems. In Switzerland one thinks of the Gotthard with its military installations from the war time. Anyone travelling in the northern part of the Japanese Alps will come across a labyrinth of tunnels of a completely different kind in the Ikenoyama Mountain. Here – protected by a 1000-metre thick rock layer from the influences of cosmic radiation and other disturbing influences – there is a collection of physical laboratories. Two of these experiments aim to uncover dark matter, one experiment investigates gravitational waves, and another the double beta decay. And then there are two experimental facilities for observing neutrinos, that class of uncharged, almost massless elementary particles that occur in three species and are very difficult to detect because they hardly interact with matter.
In search of distant supernovae
One of these two neutrino experiments is the world-famous Super-Kamiokanda detector. Since its commissioning in 1996, Super-K, how this detector is often called by physicists, has fundamentally advanced our understanding of neutrinos. In the coming months, it will be upgraded for its probably last mission: It will detect those neutrinos which have their origin in the explosion of collapsing stars (supernovae). Physicists speak of 'Supernovae Relic Neutrinos' (SRN), sometimes also of 'Diffuse Supernova Neutrino Background' (DSNB). The Super-Kamiokande can catch such neutrinos. The problem is, however, that scientists cannot distinguish these neutrinos from neutrinos of solar origin and from other galactic sources.
The aim of the Gadolinium project is to make this possible: 0.1% gadolinium is to be added to the water tank of which the Super-Kamiokande essentially consists in spring 2020. Gadolinium is a rare earth metal discovered in 1880 by the Swiss chemist Jean Charles Galissard de Marignac. According to calculations, the gadolinium detector thus upgraded will make it possible to identify a total of approximately 4 to 20 SRN – anti-electron neutrinos – over the next five years. Despite the small number, these SRNs promise important scientific findings: These neutrinos would be a new means of observing supernovae – and not only those that took place in the Milkyway, but also capable of observing extra–galactic supernovae that occur at much higher rates.
Proof of CP violation
The Super-Kamiokande promises more exciting insights over the next few years. Nevertheless, it cannot be overlooked that the detector designed in the early 1990s is now reaching its limits. And this for a simple reason, which is also known from other experiments in modern particle physics: In today's basic physics, scientific findings are often gained through the statistical evaluation of many individual events. The higher the number of these events, the more accurate the results. Exactly the same applies in neutrino physics to the question of the so-called CP violation. CP violation means that oscillations of neutrinos occur at different frequencies when compared with oscillations of their antiparticles. If this inequality is real, this might be the key to answering the fundamental question why the universe today consists practically only of matter, although the Big Bang must have produced the same amount of matter and antimatter.
In 2017, the T2K experiment, in which neutrinos produced by the J-PARC accelerator in Tokai at Japan’s east coast are detected by Super-Kamiokande, published experimental data indicating that neutrino oscillations do indeed cause CP violation. In order to obtain scientific certainty for this first observation, however, more experimental data is needed, which can only be provided by a new, large neutrino detector. "We need a bigger detector. It will be about ten times larger than the Super-Kamiokande, so in ten years it will deliver as much data as we would have to wait 100 years for with Super-Kamiokande," says Prof. Masayuki Nakahata. The Japanese neutrino physicist is director of the Kamioka Observatory, which operates Super-Kamiokande.
Microscope and telescope at the same time
The Super-Kamiokande detector is the heart of the T2K experiment, a worldwide research collaboration, also involving Switzerland, investigating the oscillations of neutrinos and antineutrinos. The successor detector will be called 'Hyper-Kamiokande' (short: 'Hyper-K'). It is to be erected eight kilometers away from Super-Kamiokande in a rock massif in which zinc and other ores were mined in the past. Construction of Hyper-K will begin in spring 2020, after the final approval by the Japanese Ministry expected in December or January. Data-taking will then start in the second half of the 2020s. It is planned to build a tank that will operate according to the Super-Kamiokanda principle, but will together contain about five times more water (260’000 tons instead of 50’000 tons) and will be equipped with significantly more photosensors (40’000 instead of 11’000, which in addition have two times better sensitivity than the current ones). The Hyper-K website describes the new research device as follows: "The Hyper-Kamiokande detector is both a ‹microscope,› used to observe elementary particles, and also a ‹telescope› for observing the Sun and supernovae, using neutrinos".
A second scientific goal of Hyper-K, besides CP violation, is the search for proton decay. This refers to the experimental confirmation of the hypothesis of the 'Great Unified Theory' (GUT), according to which protons decay (although with an extremely long half-life of longer than 10^34, the current limit set by Super-K). Hyper-K will either detect proton decay or push its current limit by another factor 5 to 10.With this goal, Hyper-K returns to the origin of particle physics research at the Kamioka site: the first detector was built there in the early 1980s with the aim of proving proton decay experimentally.
The dream of a common particle family
Prof. Nakahata hopes that proton decay will be observed with Hyper-K: "I believe that if one can find the proton decay and thus show that there is a connection between the two particle classes of hadrons (the proton is a hadron) and leptons (the positron is a lepton). That the proton - the lightest hadron - can decay into a positive electron (positron) - the lightest lepton. In this way we could show that the proton and the electron, which carry the same charge, belong to the same particle family."
Author: Benedikt Vogel