Hyper-Kamiokande (Japanese: ハイパーカミオカンデ, Hepburn: Haipā Kamiokande; also called Hyper-K or HK) is a neutrino observatory and experiment under construction in Hida, Gifu Prefecture and in Tokai, Ibaraki Prefecture in Japan. It is conducted by the University of Tokyo and the High Energy Accelerator Research Organization (KEK), in collaboration with institutes from over 20 countries across six continents. As a successor of the Super-Kamiokande (also Super-K or SK) and T2K experiments, it is designed to search for proton decay and detect neutrinos from natural sources such as the Earth, the atmosphere, the Sun and the cosmos, as well as to study neutrino oscillations of the man-made accelerator neutrino beam. The beginning of data-taking is planned for 2028. The Hyper-Kamiokande experiment facility will be located in two places:
The neutrino beam will be produced in the accelerator complex J-PARC and studied by the set of near and intermediate detectors located in Tokai village, Ibaraki prefecture, on the east coast of Japan. The main detector, also called Hyper-Kamiokande (HK), is being constructed under the peak of Nijuugo Mountain in Hida city, Gifu Prefecture, in the Japanese Alps. The HK detector will be used for proton decay searches, studies of neutrinos from natural sources and will serve as a far detector for the measurement of the oscillations of an accelerator neutrino beam at the distance corresponding to the first oscillation maximum.
Physics program
Accelerator and atmospheric neutrino oscillations Neutrino oscillations are a quantum mechanical phenomenon in which neutrinos change their flavour (neutrino flavours states: νe, νμ, ντ) while moving, caused by the fact that the neutrino flavour states are a mixture of the neutrino mass states (ν1, ν2, ν3 mass states with masses m1, m2, m3, respectively). The oscillation probabilities depend on the six theoretical parameters:
three mixing angles (θ12, θ23 and θ13) governing the mixing between mass and flavour states, two mass squared differences (∆m221 and ∆m232, where ∆m2ij = m2i – m2j) one phase (δCP) responsible for the matter-antimatter asymmetry (CP symmetry violation) in neutrino oscillations, and two parameters which are chosen for a particular experiment:
neutrino energy baseline – the distance travelled by neutrinos at which oscillations are measured. Continuing studies done by the T2K experiment, the HK far detector will measure the energy spectra of electron and muon neutrinos in the beam (produced at J-PARC as an almost pure muon neutrino beam) and compare it with the expectation in case of no oscillations, which is initially calculated based on neutrino flux and interaction models and improved by measurements performed by the near and intermediate detectors. For the HK/T2K neutrino beam peak energy (600 MeV) and the J-PARC – HK/SK detector distance (295 km), this corresponds to the first oscillation maximum, for oscillations driven by ∆m232. The J-PARC neutrino beam will run in both neutrino- and antineutrino-enhanced modes separately, meaning that neutrino measurements in each beam mode will provide information about muon (anti)neutrino survival probability Pνμ → νμ, Pνμ → νμ, and electron (anti)neutrino appearance probability Pνμ → νe, Pνμ → νe , where Pνα → Pνβ is the probability that a neutrino originally of flavour α will be observed later as having flavour β.
Comparison of the appearance probabilities for neutrinos and antineutrinos (Pνμ → νe versus Pνμ → νe) allows measurement of the δCP phase. δCP ranges from −π to +π (from −180° to +180°), and 0 and ±π correspond to CP symmetry conservation. After 10 years of data taking, HK is expected to confirm at the 5σ confidence level or better if CP symmetry is violated in the neutrino oscillations for 57% of possible δCP values. CP violation is one of the conditions necessary to produce the excess of matter over antimatter at the early universe, which forms now our matter-built universe. Accelerator neutrinos will be used also to enhance the precision of the other oscillation parameters, |∆m232|, θ23 and θ13, as well as for neutrino interaction studies. In order to determine the neutrino mass ordering (whether the ν3 mass eigenstate is lighter or heavier than both ν1 and ν2), or equivalently the unknown sign of the ∆m232 parameter, neutrino oscillations must be observed in matter. With HK beam neutrinos (295 km, 600 MeV), the matter effect is small. In addition to beam neutrinos, the HK experiment studies atmospheric neutrinos, created by cosmic rays colliding with the Earth's atmosphere, producing neutrinos and other byproducts. These neutrinos are produced at all points on the globe, meaning that HK has access to neutrinos that have travelled through a wide range of distances through matter (from a few hundred metres to the Earth's diameter). These samples of neutrinos can be used to determine the neutrino mass ordering. Ultimately, a combined beam neutrino and atmospheric neutrino analysis will provide the most sensitivity to the oscillation parameters δCP, |∆m232|, sgn ∆m232, θ23 and θ13.
Neutrino astronomy and geoneutrinos Core-collapse supernova explosions produce great quantities of neutrinos. For a supernova in the Andromeda Galaxy, 10 to 16 neutrino events are expected in the HK far detector. For a galactic supernova at a distance of 10 kpc about 50,000 to 94,000 neutrino interactions are expected during a few tens of seconds. For Betelgeuse at the distance 0.2 kpc, this rate could reach up to 108 interactions per second and such a high event rate was taken into account in the detector electronics and data acquisition (DAQ) system design, meaning that no data would be lost. Time profiles of the number of events registered in HK and their mean energy would enable testing models of the explosion. Neutrino directional information in the HK far detector can provide an early warning for the electromagnetic supernova observation, and can be used in other multi-messenger observations. Neutrinos cumulatively produced by supernova explosions throughout the history of the universe are called supernova relic neutrinos (SRN) or diffuse supernova neutrino background (DSNB) and they carry information about star formation history. Because of a low flux (few tens/cm2/sec.), they have not yet been discovered. With ten years of data taking, HK is expected to detect about 40 SRN events in the energy range 16–30 MeV. For the solar νe's, the HK experiment goals are:
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