Hyper-Kamiokande

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 square differences (∆m²₁₂ and ∆m²₃₂, where ∆m²_ij = m²_i - m²_j) and δ_CP phase responsible for the matter-antimatter asymmetry in the neutrino oscillations.[5]^{: 285–311}[2]^: 20–23

Continuing studies done by the T2K experiment, the HK detector will measure the energy spectra of electron and muon neutrinos in the beam produced at J-PARC as almost pure muon neutrino beam and compare it with the beam composition measured by the near and intermediate detectors. For the T2K/HK neutrino beam mean energy, the J-PARC - HK detector distance corresponds to the first oscillation maximum. The measurements will be done separately for the neutrino and antineutrino beams, providing information about P
_{ν
μ →
ν
e}, P
_{ν
μ →
ν
μ}, P
_{ν
μ →
ν
e}, P
_{ν
μ →
ν
μ}, where P_να → P_νβ is the probability that a neutrino originally of flavour α will be observed later as having flavour β.[2]^{: 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 to measure δ_CP phase. δ_CP ranges from -π to π (from −180° to 180°), and 0 and ±π correspond to the CP symmetry conservation. After 10 years of data taking, HK is expected to confirm at the 5σ confidence level if CP symmetry is violated in the neutrino oscillations for 57% of possible δ_CP values.[2] The 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 to enhance also the precision of the other oscillation parameters, |∆m²₃₂|, θ₂₃ and θ₁₃, as well as for the neutrino interaction studies.[2]^{: 202–224}

The unknown sign of the ∆m²₃₂ parameter manifests itself only during the neutrino propagation in matter. The HK experiment studies of the atmospheric neutrinos travelling through the Earth and entering the HK detector from different directions (the baseline in the matter up to the Earth's diameter) will be used to measure it.[2]^{: 225–237}

Supernova explosions produce great quantities of neutrinos. For a galactic supernova at a distance of 10 kpc about 50000 to 94000 neutrino interactions are expected in the HK detector 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 number of events was taken into account during the design of the detector DAQ system. 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 detector can provide an early warning for the electromagnetic supernova observation, as well as can be used in other multi-messenger observations.[6][2]^{: 263–280}

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 low flux (few tens/cm²/sec.), they are not discovered yet. For ten years of data taking, HK is expected to detect about 70 SRN events for the energy range 16-30 MeV.[2]^{: 276–280}[7]

For the solar
ν
_e's, the HK experiment goals are i.a.:

• Search for day-night asymmetry in the neutrino flux - resulting from different distances travelled in matter (during the night neutrinos cross the Earth before entering the detector) and thus the different oscillation probabilities caused by the matter effect.[2]^{: 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.[2]^{: 238–244}[8]
• The first observation of neutrinos from the hep channel: ${\displaystyle {^{3}}{\text{He}}+{\text{p}}\to {^{4}}{\text{He}}+{\text{e}}^{+}+\operatorname {\nu } _{\text{e}}}$ predicted by the Standard Solar Model.[2]^{: 238–244}
• Comparison of the neutrino flux with the solar activity (e.g. the 11-year solar cycle).[9]

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.[2]^{: 292–293}

The decay of a free proton into lighter subatomic particles has never been observed, but it is predicted by some grand unified theories (GUT) and was first proposed to explain the predominance of matter over antimatter in the universe. The main channels studied by HK are
p⁺
→
e⁺
+
π⁰
and
p⁺
→
ν
+
K⁺
, which are favoured by many GUT models. After ten years of data taking, HK is expected to increase the lower limit of the proton mean lifetime from 1.6x10³⁴ to 6.3x10³⁴ for its most sensitive decay channel (
p⁺
→
e⁺
+
π⁰
) and from 0.7x10³⁴ to 2.0x10³⁴ for another decay channel (
p⁺
→
ν
+
K⁺
).[2]^{: 26–28, 245–257}

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.[2]^{: 281–286}

The Intermediate Water Cherenkov Detector (IWCD) will be located at a distance of 0.7–2.0 kilometres (0.43–1.24 mi) from the neutrino production place. It would 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 3000 photomultiplier tubes of a 20 centimetres (7.9 in) diameter. The structure will be moved in a vertical direction by a crane system, providing measurements of neutrino interactions at different off-axis angles, spanning from 1° at the bottom to 4° at the top, and thus for different energy spectra. Combining the results from different off-axis angles it is possible to extract the results for nearly monochromatic 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 comparing results from these two detectors without relying on detectors response simulations. These two facts, independence from the neutrino interaction and detector response models, will enable to minimise systematic error in the oscillation analysis. Additional advantages of such a design of the detector is a possibility to search for sterile oscillation pattern for different off-axis angles and to obtain a cleaner sample of electron neutrino interaction, whose fraction is larger for the larger off-axis angle.[2]^: 47–50[11][12]

Schematics of the Hyper-Kamiokande water Cherenkov detector.

The Hyper-Kamiokande detector will be located 8 kilometres (5.0 mi) south from the Super-Kamiokande (SK) detector in the Tochibora mine, 650 metres (2,130 ft) under the peak of Nijuugo Mountain, 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 as SK.

Prototype of mPMT for the Hyper-Kamiokande Inner Detector.

Scheme of multiPMT for the Hyper-Kamiokande detector.

HK will be a water Cherenkov detector, 5 times larger (258 kton of water) than the SK detector. It will be a cylindrical tank of 74 metres (243 ft) diameter and 60 metres (200 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 (PMT) looking both inwards to the ID and outwards to the OD. For ID, there will be up to 40000 of 50 centimetres (20 in) diameter PhotoMultiplier Tubes (PMT) of type R12860 by Hamamatsu Photonics and approximately 1000 multi-PMT modules (mPMTs). Each mPMT module consists of nineteen 3-inch diameter photomultiplier tubes encapsulated in a water-proof vessel. The OD will be instrumented with up to 8000 7.6 centimetres (3.0 in) PMTs and will serve as a veto[note 1] to distinguish interactions occurring inside from particles entering from the outside of the detector (mainly cosmic-ray muons).[13][14]

J-PARC neutrino beam Japan to Korea

The HK detector construction began in 2020 and the start of data collection is expected in 2027.[10]^: 24[2][3] It is also considered to build a second, identical water-Cherenkov tank in Korea around 1100 km from J-PARC, which would be operational 6 years after the first tank.[4]