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The T2K Experiment

The proton beams extracted from the main ring synchrotron at J-PARC are directed in a westward direction through the T2K primary beam line. The beams strike a target composed of graphite rods (see figure below), and produce a large number of positively charged pions. The directions of motion of these pions are made to converge in the forward direction by some magnetic horns.

The pions then decay into muons and muon neutrinos in a 100-metre-long tunnel known as the decay volume (see figure).

The muons and any remaining pions are stopped by a second layer of graphite, while the muon neutrinos pass through this layer. The composition of the muon-neutrino beam is measured by a near detector located 280 metres downstream of the target (see figure below).

Neutrino oscillations are studied by comparing the composition of the muon-neutrino beam at the near detector with its composition at Super Kamiokande, which is 295 km from the target.

About Super Kamiokande

Super Kamiokande is the world's largest underground neutrino detector (represented in figure below), and is located 1000 metres underground in Kamioka Mine, Hida, Gifu Precture, Japan. It is affiliated with the Kamioka Observatory of the Institute of Cosmic Ray Research at the University of Tokyo. In addition to detecting T2K neutrinos, Super Kamiokande observes neutrinos produced by collisions between cosmic rays and molecules in the Earth's upper atmosphere. It is also searching for proton decays, which have never been observed to date. Super Kamiokande consists of a large cylinder 39.3 metres in diameter and 41 metres high that contains 50,000 tons of ultra-pure water. The inner walls of the cylinder are lined with about 11,200 photomultiplier tubes to detect Cerenkov light, which is emitted when a charged particle travels faster than the speed of light in water (this is three-quarters of its speed in vacuum). 

In this 3D image below of the cylindrically-shaped Super Kamiokande, each coloured dot represents a photomultiplier that detected light. An electron neutrino interacts with a neutron in a nucleus of a water molecule to produce an electron and a proton. The electron often travels faster than the speed of light in water, and causes Cerenkov light to be emitted from the water atoms. This light is seen as a ring by the photomultipliers of Super Kamiokande. The image shows the first electron-neutrino candidate observed after the recovery from the earthquake on the east coast of Japan in 2011.

About Neutrinos and Neutrino Oscillations

Neutrinos are elementary particles that come in three types: electron neutrinos, muon neutrinos and tau neutrinos. They are electrically neutral, and their masses are not known but are believed to be of the order of one millionth of the masses of quarks and electrons.

Neutrinos change from one type to another as they travel. This change of type is called “neutrino oscillation”, and it can only happen if neutrinos have different masses. It was first predicted by Pontecorvo, Maki, Nakagawa and Sakata in 1962. Oscillations can take place between any of the three types of neutrino and any other type.

Photos and text on this page courtesy of © The T2K Collaboration 2013