Detection method

The Super-Kamiokande experiment mainly observes neutrinos using a huge water tank with approximately 13,000 PMTs.

When a neutrino enters the detector and interacts with water, Cherenkov lights emitted.

50,000-ton water target

One of the goals of the Super-Kamiokande experiment is to elucidate the whole nature of neutrinos through observations of solar neutrinos, atmospheric neutrinos, and artificial neutrinos. In 1998, Super-Kamiokande discovered a phenomenon in which neutrinos change their type during their flight (neutrino oscillation) by observations of atmospheric neutrinos. In 2001, observations of solar neutrinos led to the discovery of solar neutrino oscillations, and in 2011, artificial neutrinos led to the discovery of a third oscillation mode. Elucidating the properties of neutrinos will lead us to the mystery of how matter was created in the early universe.

13,000 eyes watch Cherenkov light

When the generated charged particle in water has a higher velocity than the speed of light in water, Cherenkov light is emitted. This phenomenon is similar to that of the diagonal wave, which is made on the water surface when a duck moves at a higher velocity than the speed of the water surface wave.

The Cherenkov light is emitted in a cone shape to the direction of a charged particle. The PMT on the tank’s wall detects the Cherenkov light. The PMTs have information on the amount of light detected and the timing of the detection. The information from the PMTs determines the energy, direction, interaction point, and type of the charged particles.

The generated charged particles emit Cherenkov light.

This image shows the event display of a muon neutrino detected by the Super-Kamiokande. The colored points indicate the amount of light detected by each PMT. The Cherenkov ring emitted by a muon is also shown.

Mountain as an Umbrella

Primary cosmic rays (mainly protons) are continuously poured on the Earth. When a cosmic ray interacts with the earth’s atmosphere, muons, electrons, and neutrinos (referred to as secondary cosmic rays) are generated. Many muons lose their energy and stop stop at the ground. However, neutrinos do not stop because they rarely interact with matter.

The mountain above the detector acts as an umbrella, shielding it from cosmic ray muons, which form the background of neutrino observations. The cosmic ray muons are reduced to 1/100,000 of the ground surface by 1000 m rock overburden.

Event Display

These images can be used for publication under the conditions, described here.

How to See the Event Display

The event display shows information about the light captured by the photomultiplier tubes, allowing the user to visually see the data detected by Super-Kamiokande. By looking at this display, it is possible to determine the type and direction of the detected particles roughly.

The display shows the expansion of the cylindrical detector. The larger figure at the center shows the ID with 11,129 20-inch PMTs and the smaller figure shows the data of the OD. The colored dots visualize information on which tubes got hit during the event on display. Two modes are possible: Color can encode either the charge (Q) registered at the particular tube or the time (T) when the tube was hit. The entry “Current” in the upper left corner of the display indicates which mode of the display was chosen for the particular event. More printed information on the current event can be found beneath it.

Cosmic ray Muon Events

Super-Kamiokande detects approximately 2 Hz of cosmic ray muons. Some interesting images are shown here.

Stopping muon

Corner edge clipping muon

Double muon (Timing distribution)

Double muon (Charge distribution)

Triple muons

Stopping muon entering from the top (Charge distribution)

Stopping muon entering from the top (Timing distribution)

An electron emitted from muon decay

Neutrino Events

The number of photons detected by the PMTs of the outer detector distinguishes neutrino events from cosmic ray muon events. When the cosmic ray muon enters the outer detector, the Cherenkov light is emitted immediately because the cosmic ray muon is a charged particle. The cosmic ray muon runs into the water and continues to emit Cherenkov light, which is detected by the inner PMTs.

However, because it is a neutral particle, a neutrino does not emit the Cherenkov light. The charged particles emit Cherenkov light when they interact with a neutrino in water. Therefore, in most cases of neutrino events, only the inner PMTs have hits, whereas the outer PMTs do not. The outer PMTs are extremely effective at distinguishing neutrino events from charged particles such as cosmic ray muons.

A muon neutrino generated a Cherenkov ring. A muon neutrino interacts with a nucleon in water and transforms into a muon. The outer detector has few hits in the right-upper display.

An electron neutrino event. An electron neutrino scatters an electron in water. The emitted electron generates an electromagnetic shower, resulting in a fuzzy edge of the Cherenkov ring.

Real-time Monitor

This display shows the data detected by Super-Kamiokande in real-time. (This is a test operation and subject to change without notice)

Real-time Monitor

The number of neutrinos observed in Super-Kamiokande is about 30 per day. In terms of probability, it may be nearly impossible to find neutrinos on this monitor. Almost all events are cosmic ray muon events. Look at the different light patterns and imagine where the particles came in and which direction they went out.

The following facilities have exhibits about Super-Kamiokande, including the real-time monitors.

Real-time monitors