Hyper-Kamiokande (also calledHyper-K orHK) is aneutrinoobservatory andexperiment under construction inHida,Gifu and inTokai,Ibaraki inJapan. It is conducted by theUniversity of Tokyo and theHigh Energy Accelerator Research Organization (KEK), in collaboration with institutes from over 20 countries across six continents.[1][2] As a successor of theSuper-Kamiokande (also Super-K or SK) andT2K experiments, it is designed to search forproton decay and detect neutrinos from natural sources such as theEarth, the atmosphere, theSun and the cosmos, as well as to studyneutrino oscillations of the man-madeaccelerator neutrino beam.[3]: 6, 20–28 The beginning of data-taking is planned for 2027.[4]
The Hyper-Kamiokande experiment facility will be located in two places:
Neutrino oscillations are aquantum mechanical phenomenon in which neutrinos change theirflavour (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:
and two parameters which are chosen for a particular experiment:
Continuing studies done by theT2K 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
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 5σconfidence level or better if CP symmetry is violated in the neutrino oscillations for 57% of possible δCP values. CP violation is one of theconditions 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 theneutrino 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 studiesatmospheric neutrinos, created bycosmic 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 theEarth'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
Core-collapse supernova explosions produce great quantities ofneutrinos. For a supernova in theAndromeda Galaxy, 10 to 16 neutrino events are expected in the HK far detector. For a galactic supernova at a distance of 10kpc about 50,000 to 94,000 neutrino interactions are expected during a few tens of seconds. ForBetelgeuse 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 detectorelectronics anddata 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 othermulti-messenger observations.[3]: 263–280 [7]
Neutrinos cumulatively produced by supernova explosions throughout the history of the universe are called supernova relic neutrinos (SRN) ordiffuse 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]
For thesolar
ν
e's, the HK experiment goals are:
Geoneutrinos are produced in decays ofradionuclides inside the Earth. Hyper-Kamiokande geoneutrino studies will help assess theEarth's core chemical composition, which is connected with the generation of thegeomagnetic field.[3]: 292–293
Thedecay of a freeproton into lightersubatomic particles has never been observed, but it is predicted by somegrand unified theories (GUT) and results frombaryon number (B) violation. B violation is one of the conditions needed to explain thepredominance ofmatter overantimatter in theuniverse. The main channels studied by HK are
p+
→
e+
+
π0
which is favoured by many GUT models and
p+
→
ν
+
K+
predicted by theories includingsupersymmetry.[11]
After ten years of data taking, (in case no decay will be observed) HK is expected to increase the lower limit of the protonmean lifetime from 1.6 · 1034 to 6.3 · 1034 years for its most sensitive decay channel (
p+
→
e+
+
π0
) and from 0.7 · 1034 to 2.0 · 1034 years for the
p+
→
ν
+
K+
channel.[3][12]
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 interactweakly, they may produce neutrinos throughannihilation or decay. Those neutrinos could be visible in the HK detector as an excess of neutrinos from the direction of largegravitational potentials such as thegalactic centre, theSun or theEarth, over an isotropicatmospheric neutrino background.[3]: 281–286
The Hyper-Kamiokande experiment consists of anaccelerator 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 forproton decay searches and studies of neutrinos from natural sources. All the above elements will serve for the acceleratorneutrino 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 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) diameterPhotoMultiplier 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 differentoff-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 forsterileoscillation patterns for different off-axis angles and to obtain a cleaner sample ofelectron neutrino interactions, whose fraction is larger for larger off-axis angles.[3]: 47–50 [14][15][16][17]
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 theSuper-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 inJ-PARC.[note 2][3]: 35 [18]
HK will be awaterCherenkov detector, 5 times larger (258 kton of water) than the SK detector. It will be acylindrical 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 holdPhotoMultiplier Tubes (PMTs) looking both inwards to the ID and outwards to the OD.[18][19]
In the ID, there will be at least 20,000 50 centimetres (20 in) diameterPhotoMultiplier Tubes (PMT) of R12860 type byHamamatsu 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 3,600 8 centimetres (3.1 in) diameter PMTs coupled with 0.6×30×30 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 (mainlycosmic-ray muons).[18][19][17]
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 inSouth Korea around 1100 km from J-PARC, which would be operational 6 years after the first tank.[5][20]
A history of large water Cherenkov detectors in Japan, and long-baseline neutrino oscillation experiments associated with them, excluding HK:
A history of the Hyper-Kamiokande experiment:
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