The Hyper Kamiokande Experiment
Hyper Kamiokande (Hyper-K) is a proposed next generation neutrino observatory. The one million tonnes of water in the detector will detect neutrinos from a man made neutrino beam, the Sun, the atmosphere and the cosmos. Hyper-K will give us a deeper insight into our Universe at the largest and smallest scales.
Neutrinos see the majority of the atoms around us as empty space. They do not have an electric charge and therefore only interact through what is called the weak nuclear force. This fundamental force of Nature has a very short range, no greater than the size of an atomic nucleus. The nucleus of an atom is where most of the mass of an atom can be found and so it is the most likely place for a neutrino to interact with an atom. But, the nucleus is 10,000 times smaller than the atom itself. This means that to a neutrino 99.99% of an atom is essentially empty space because it cannot interact with it.
Even if a neutrino does come into contact with the 0.001% of the atom that is the nucleus it is still a game of chance as to whether the neutrino will interact or not. To give an idea of how rarely they interact let us consider one source of neutrinos: The Sun. Right now there are around 200,000,000,000 neutrinos flowing through your
body from the Sun every single second. Even if you lived to the ripe old age of 80 years old (~43,000,000 seconds) you would be lucky if even one neutrino even noticed that you existed by interacting with an atom in your body. They are extremely ghostly.
So how do we see them? Well it is all about increasing the odds of catching a neutrino on a very widely spaced net of atomic nuclei. We can do two things either get more neutrinos passing through the net or make the holes in the net smaller. The first solution requires us to create a man made beam of neutrinos, which is part of the Hyper-K program of activities. The second solution requires us to build the biggest detectors that we can with as much stuff for neutrinos to interact with as possible; Hyper-K is the biggest neutrino detector ever proposed.
Hyper-K is designed as a scaled up version of the extremely successful Super Kamiokande (Super-K) detector. The tried and tested technologies used in Super-K will be replicated on a much larger scale. Super-K contained 50,000 metric tonnes of water as a net in which to catch neutrinos and Hyper-K will have 20 times this amount, weighing in at 1,000,000 tonnes! The water will be contained in two large cylinders lying side-by-side; 250 m long, 54 m high, and 48 m wide. These cylinders will be caverns cut into rock underneath the mountains in western Japan located just 8 km south of its Super-K predecessor. With a net of this size Hyper-K offers improvement in every area of our understanding of neutrinos.
Hyper-K is more than a detector it is an observatory to look at neutrinos coming from many different sources. Each neutrino source can tell us a wealth of information. Hyper-K will be able to give us an insight into the creation story of the Universe from an understanding of why the matter making up every atom in our body is preferred over it’s opposite number antimatter. This understanding will come from precise measurements of a phenomenon known as neutrino oscillations. Neutrinos from the Sun, atmosphere, and a man made beam will offer answers to this and other long-standing questions.
Hyper-K will also forge ahead in the field of neutrino astronomy. Light from distant sources in the cosmos can be absorbed by clouds of dust or deflected by large galactic magnetic fields. Neutrinos on the other hand fly straight and true direct from the same cosmic sources. Supernova explosions are the violent death of massive stars and the source of much of the heavier chemical elements in our universe. They can outshine the galaxy in which they reside but this is just 0.01% of the overall energy release in such an explosion; 99% of the energy is released in the form of neutrinos. If a Supernova occurred within our galaxy then Hyper-K would detect 50,000-500,000,000 neutrinos, depending on how distant the Supernova is, in just 10 seconds. This would give us unprecedented information about the death of stars and their enrichment of heavy elements.
Hyper-K will also continue the work of Super-K when trying to observe the decay of protons. As far as we understand it the proton is stable; it does not decay as there is no similar particle lighter in mass. There are processes, however, in exotic theories that allow the proton to decay. These theories explain a common origin to the forces of nature in the very young Universe and are called Grand Unified Theories. With a common origin it is rare but possible that the proton could decay and in it death produce a positron (antielectron) and a neutral pion (a meson, quark-antiquark pair). The more atoms there are in a detector the more protons will be around that might decay. As Hyper-K will be the largest ever particle physics detector it will contain the most protons and can put the best limit on the possible decay lifetime of the proton.