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Experimental Technique

Cherenkov lightThe Hyper-Kamiokande experiment uses a huge amount of water as a target for neutrinos. Due to the fact that neutrinos interact only very weakly with matter, they are very difficult to detect. However when neutrinos undergo a rare interaction with a nucleus or electron, electron neutrinos eject electrons and muon neutrinos eject muons. Electrons and muons have a charge, and so Cherenkov light is emitted.

写真Cherenkov light is emitted when the velocity of a charged particle is higher then the velocity of light. The velocity of light is slower in water than in vacuum, as it is affected by the refractive index of the water.

Cherenkov light is emitted in a cone shape, along the direction of the charged particle, and therefore the energy, direction and type of the neutrinos are determined using certain information obtained from the photosensors, such as the amount of light detected and the ring shape.

Why is the detector underground?

Cosmic rays consisting mainly of protons are incident on the surface of the earth. When these cosmic rays collide with the earth’s atmosphere, muons, electrons and neutrinos are generated and are poured onto the earth’s surface. These muons disturb the observation of neutrinos.

In an underground setting, most of the muons have lost their energy and have been stopped. For example, at a depth of 1000 m, the amount of cosmic ray muons is decreased to only 0.001% of the amount incident on the earth’s surface. On the other hand, neutrinos are able to pass through the earth in order to reach the detector, since neutrinos rarely interact with matter.

In this way, thanks to the umbrella effect of the ground in shutting out the obstacle of muons, the detection of neutrinos is enabled.