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 (36°21′20.105″N137°18′49.137″E / 36.35558472°N 137.31364917°E / 36.35558472; 137.31364917[3]: 56 ). 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.[3]: 53–56 [5]
Physics program
Accelerator and atmospheric neutrino oscillations
Neutrino oscillations are a quantum mechanical phenomenon in which neutrinos change their flavour (neutrino flavours states: ν e, ν μ, ν τ) 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.[6]: 285–311 [3]: 20–23
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 P ν μ → ν μ, P ν μ → ν μ, and electron (anti)neutrino appearance probability P ν μ → ν e, P ν μ → ν e , where Pνα → Pνβ is the probability that a neutrino originally of flavour α will be observed later as having flavour β.[3]: 202–224
The ability of Hyper-K to exclude CP conservation as a function of the true value of δCP
Comparison of the appearance probabilities for neutrinos and antineutrinos (P ν μ → ν e versus P ν μ → ν e) 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.[3]: 202–224
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.[3]: 225–237
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.[3]: 228–233
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 50000 to 94000 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.[3]: 263–280 [7]
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.[3]: 276–280 [8]
Search for a day-night asymmetry in the neutrino flux – resulting from different distances travelled in matter (during the night neutrinos additionally cross the Earth before entering the detector) and thus the different oscillation probabilities caused by the matter effect.[3]: 238–244
Measurement of the ν e survival probability for neutrino energies between 2 and 7 MeV – i.e. between regions dominated by oscillations in vacuum and oscillations in matter, respectively – which is sensitive to new physics models, like sterile neutrinos or non-standard interactions.[3]: 238–244 [9]
The first observation of neutrinos from the hep channel: predicted by the standard solar model.[3]: 238–244
Comparison of the neutrino flux with the solar activity (e.g. the 11-year solar cycle).[10]
Geoneutrinos are produced in decays of radionuclides inside the Earth. Hyper-Kamiokande geoneutrino studies will help assess the Earth's core chemical composition, which is connected with the generation of the geomagnetic field.[3]: 292–293
After ten years of data taking, (in case no decay will be observed) HK is expected to increase the lower limit of the proton mean lifetime from 1.6x1034 to 6.3x1034 years for its most sensitive decay channel ( p+ → e+ + π0 ) and from 0.7x1034 to 2.0x1034 years for the p+ → ν + K+ channel.[3][12]
Dark Matter
Dark matter is a hypothetical, non-luminous form of matter proposed to explain numerous astronomical observations suggesting the existence of additional invisible mass in galaxies. If the dark matter particles interact weakly, they may produce neutrinos through annihilation or decay. Those neutrinos could be visible in the HK detector as an excess of neutrinos from the direction of large gravitational potentials such as the galactic centre, the Sun or the Earth, over an isotropic atmospheric neutrino background.[3]: 281–286
Experiment Description
The Hyper-Kamiokande experiment consists of an accelerator neutrino beamline, a set of near detectors, the intermediate detector and the far detector (also called Hyper-Kamiokande).
The far detector by itself will be used for proton decay searches and studies of neutrinos from natural sources. All the above elements will serve for the accelerator neutrino oscillation studies. Before launching the HK experiment, the T2K experiment will finish data taking and HK will take over its neutrino beamline and set of near detectors, while the intermediate and the far detectors have to be constructed anew.[13]
The muon neutrino flux at the IWCD detector for different off-axis angles
The electron neutrino flux at the IWCD detector for different off-axis angles
The Intermediate Water Cherenkov Detector (IWCD) will be located at a distance of around 750 metres (2,460 ft) from the neutrino production place. It will be a cylinder filled with water of 10 metres (33 ft) diameter and 50 metres (160 ft) height with a 10 metres (33 ft) tall structure instrumented with around 400 multi-PMT modules (mPMTs), each consisting of nineteen 8 centimetres (3.1 in) diameter PhotoMultiplier Tubes (PMTs) encapsulated in a water-proof vessel. The structure will be moved in a vertical direction by a crane system, providing measurements of neutrino interactions at different off-axis angles (angles to the neutrino beam centre), spanning from 1° at the bottom to 4° at the top, and thus for different neutrino energy spectra.[note 1]Combining the results from different off-axis angles, it is possible to extract the results for nearly monoenergetic neutrino spectrum without relying on theoretical models of neutrino interactions to reconstruct neutrino energy. Usage of the same type of detector as the far detector with almost the same angular and momentum acceptance allows comparison of results from these two detectors without relying on detector response simulations. These two facts, independence from the neutrino interaction and detector response models, will enable HK to minimise systematic error in the oscillation analysis. Additional advantages of such a design of the detector is the possibility to search for sterileoscillation patterns for different off-axis angles and to obtain a cleaner sample of electron neutrino interactions, whose fraction is larger for larger off-axis angles.[3]: 47–50 [14][15][16][17]
Hyper-Kamiokande Far Detector
Schematic of the Hyper-Kamiokande Far Detector, a water Cherenkov detector
The Hyper-Kamiokande detector will be built 650 metres (2,130 ft) under the peak of Nijuugo Mountain in the Tochibora mine, 8 kilometres (5.0 mi) south from the Super-Kamiokande (SK) detector. Both detectors will be at the same off-axis angle (2.5°) to the neutrino beam centre and at the same distance (295 kilometres (183 mi)) from the beam production place in J-PARC.[note 2][3]: 35 [18]
Mockup with 50 cm R12860 PMTs for Hyper-Kamiokande Far Detector Inner Detector
Prototype of a mPMT for the Hyper-Kamiokande Far Detector Inner Detector
Schematic of a mPMT for the Hyper-Kamiokande Far Detector Inner Detector
3-inch PMT (Photomultiplier) and WLS (Wavelength-Shifting Fiber) plate for Hyper-Kamiokande Far Detector Outer Detector
HK will be a waterCherenkov detector, 5 times larger (258 kton of water) than the SK detector. It will be a cylindrical tank of 68 metres (223 ft) diameter and 71 metres (233 ft) height. The tank volume will be divided into the Inner Detector (ID) and the Outer Detector (OD) by a 60 cm-wide inactive cylindrical structure, with its outer edge positioned 1 meter away from vertical and 2 meters away from horizontal tank walls. The structure will optically separate ID from OD and will hold PhotoMultiplier Tubes (PMTs) looking both inwards to the ID and outwards to the OD. In the ID, there will be at least 20000 50 centimetres (20 in) diameter PhotoMultiplier Tubes (PMT) of R12860 type by Hamamatsu Photonics and approximately 800 multi-PMT modules (mPMTs). Each mPMT module consists of nineteen 8 centimetres (3.1 in) diameter photomultiplier tubes encapsulated in a water-proof vessel. The OD will be instrumented with at least 3600 8 centimetres (3.1 in) diameter PMTs coupled with 0.6x30x30 cm3wavelength shifting (WLS) plates (plates will collect incident photons and transport them to their coupled PMT) and will serve as a veto[note 3] to distinguish interactions occurring inside from particles entering from the outside of the detector (mainly cosmic-ray muons).[18][19][17]
J-PARC neutrino beam Japan to Korea
HK detector construction began in 2020 and the start of data collection is expected in 2027.[3][4][13]: 24 Studies have also been undertaken on the feasibility and physics benefits of building a second, identical water-Cherenkov tank in South Korea around 1100 km from J-PARC, which would be operational 6 years after the first tank.[5][20]
History and schedule
Hyper-Kamiokande detector construction schedule
A history of large water Cherenkov detectors in Japan, and long-baseline neutrino oscillation experiments associated with them, excluding HK:
1996–present: Super-Kamiokande experiment – the predecessor of the Hyper-Kamiokande experiment, studying neutrinos from natural sources and searching for proton decay (the Nobel Prize in Physics 2015 for Takaaki Kajita)[1]
January 2015: MoU for cooperation in the Hyper-Kamiokande project signed by two host institutions: ICRR and KEK. Formation of the Hyper-Kamiokande proto-collaboration[24][25]
^The average energy of neutrinos decreases with the deviation from the beam axis.
^The Super-Kamiokande detector serves as a far detector for the neutrino oscillation analysis by the T2K experiment. However, Super-Kamiokande is also a separate experiment in the matter of proton decay searches and studies of neutrinos from natural sources.
^Veto is part of a detector where no activity should be registered to accept an event. Such a requirement allows constraining the number of background events in a selected sample.
^
Mine, Shunichi (2023). "Nucleon decay: theory and experimental overview". Zenodo. doi:10.5281/zenodo.10493165.
^K. S. Babu; E. Kearns; et al. (2013-11-20). "Baryon Number Violation". Proceedings, 2013 Community Summer Study on the Future of U.S. Particle Physics: Snowmass on the Mississippi (CSS2013). Minneapolis, MN, USA. arXiv:1311.5285.
^nuPRISM Collaboration (13 December 2014). "Letter of Intent to Construct a nuPRISM Detector in the J-PARC Neutrino Beamline". arXiv:1412.3086 [physics.ins-det].
^Shiozawa, M. (23–25 September 1999). "Study of 1-Megaton water Cherenkov detectors for the future proton decay search". AIP Conf.Proc. 533 (2000) 1, 21–24. International Workshop on Next Generation Nucleon Decay and Neutrino Detector (NNN99). Stony Brook, NY, United States. doi:10.1063/1.1361719.
^K. Abe; et al. (15 September 2011). "Letter of Intent: The Hyper-Kamiokande Experiment --- Detector Design and Physics Potential ---". arXiv:1109.3262 [hep-ex].
^Itow, on behalf of the Hyper-Kamiokande Collaboration, Y. (2021). "Construction status and prospects of the Hyper-Kamiokande project". Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021). Proceedings of Science. p. 1192. doi:10.22323/1.395.1192. S2CID199687331.
