T2K (Tokai to Kamioka) is a long-baseline neutrino experiment in Japan, and is studying neutrino oscillations. Neutrinos are elementary particles which come in three “flavours”: electron, muon, and tau. They only interact through the weak force, and are very difficult to detect as they rarely interact with matter. Electron neutrinos are produced in large numbers in the Sun, and solar neutrinos can pass all the way through the Earth without interacting.
T2K has made a search for oscillations from muon neutrinos to electron neutrinos, and announced the first experimental indications for them in June 2011. These oscillations had never been observed by any previous experiment. T2K is also making measurements of oscillations from muon neutrinos to tau neutrinos (which have been seen by previous experiments). It will make the most accurate measurements to date of the probability of these oscillations and of the difference between the masses of two of the neutrinos (to be precise, T2K measures the difference between the squares of these masses).
The T2K Neutrino Beam
The T2K experiment sends an intense beam of muon neutrinos from Tokai, which is on the east coast of Japan, to Kamioka at a distance of 295 km in western Japan. The neutrino beam is made in collisions between a proton beam and a graphite target; these collisions produce pions, which quickly decay to muons and muon neutrinos. The muons and any remaining protons and pions are stopped by a second layer of graphite, but the neutrinos pass through this layer. The energy of the neutrinos in the beam is important as oscillations depend on it: low-energy neutrinos oscillate in a shorter distance than high-energy neutrinos. The T2K neutrino beam has a range of energies centred on 600 MeV since muon neutrinos with this energy are most likely to oscillate after travelling 295 km.
T2K has also started (in 2014) taking data using a muon-antineutrino beam. It is believed that equal quantities of matter and antimatter were produced in the Big Bang and it is not understood why the present-day Universe is composed entirely of matter. The aim of using an antineutrino beam will be to seek a solution to this by comparing antineutrino oscillations with neutrino oscillations.
The T2K Detectors
It is essential that the direction of the neutrino beam be stable to within 1/20 th of a degree, and that the intensity of the beam be constant over time. Checks of the direction and intensity of the beam are made on a daily basis using neutrino interactions with iron in the Interactive Neutrino GRID (INGRID) near detector. This is situated 280 metres from the target in the centre of the neutrino beam.
T2K studies neutrino oscillations with two separate detectors, both of which are 2.5 degrees away from the centre of the neutrino beam. The ND280 near detector is also 280 metres from the target, and measures the number of muon neutrinos in the beam before any oscillations occur. T2K neutrinos have much higher energies than solar neutrinos, and high-energy neutrinos are more likely to interact. A small number of muon neutrinos interact with scintillator or water in the ND280, and many of these interactions produce a muon. The muon is a charged particle, and can be detected since it ionises gas which is placed immediately after the interaction points. These ND280 measurements are used to predict the number of muon neutrinos that would be seen in the “far detector” SuperKamiokande if there were no oscillations.
Most of the neutrinos pass through the ND280 without interacting, and these travel at near the speed of light to Super Kamiokande (Super K). This is located 1000 metres underground in western Japan, and is 295 km from the target in Tokai. At the Super K detector, the neutrinos enter a very large cylinder of ultra-pure water. Again most of the neutrinos pass through without interacting but, due to the high energies of the neutrinos and the intensity of the beam, some do interact with the water.
Many of the interactions of muon neutrinos produce muons, while interactions of electron neutrinos often produce electrons. Muons and electrons are charged particles, and they displace electrons in the water as they pass. As the water electrons return to their equilibrium positions after the passage of the charged particle, they emit light. If the passing charged particle is travelling faster than the velocity of light in water (which is three-quarters of its velocity in a vacuum), this light is emitted as a cone known as Cerenkov radiation. The walls of Super K are lined with more than 10,000 sensitive photo-multipliers, which detect the cone of Cerenkov light as a ring. Super K can distinguish muons (which produce a sharp ring) from electrons (which produce a more diffuse ring).
For an almost-live event display from Super-K, click here. You will see a map of the phototubes inside the Super-K detector, from an event recorded a short time ago. The display will update every few seconds. These are just a random sample of the events recorded every second at Super-K. (For an explanation of the event display, click here). Most of the events you see will be downward-going muon tracks, NOT T2K beam neutrinos! In fact most of the neutrino interactions we sift out of the millions of events recorded at Super-K are caused by neutrinos from the Sun or the Earth’s atmosphere. Only a few hundred neutrino interactions per year are due to beam neutrinos traveling from J-PARC.
The oscillations from muon neutrinos to electron neutrinos were seen in Super K as the diffuse rings from electrons produced in interactions of electron neutrinos with the water. 28 electron-neutrino events have been seen in Super K, whereas only 4.6 would have been expected if there were no oscillations. The probability that these 28 events were due to a process other than oscillations from muon neutrinos to electron neutrinos is tiny at 10-13, and confirms that these oscillations are occurring.
T2K is also studying oscillations from muon neutrinos to tau neutrinos, and these are seen as a reduction in the number of muon neutrinos detected in Super K compared with the ND280 prediction for no oscillations.
Advantages of an Off-Axis Experiment
T2K is the world’s first off-axis neutrino experiment, with ND280 and Super K positioned 2.5 degrees away from the centre of the neutrino beam.
The off-axis part of the beam has a narrower range of energies than the on-axis part, which means that a larger fraction of neutrinos change flavor by the time they reach Super K. Also the most important measurement is that of neutrino energy and this measurement is made most accurately from events in which a neutrino interacts with a neutron in the detector to produce a muon and a proton. The off-axis part of the beam has a larger fraction of these events than the on-axis part, which enables T2K to make more accurate measurements of neutrino energy. This leads to more accurate measurements of the probability of neutrino oscillations and neutrino mass differences than those from previous experiments.
The T2K Schedule
T2K began its experimental commissioning in April 2009 and to take physics data in January 2010. It suspended its operation after the major earthquake of March 2011 in order to check and repair its equipment. These repairs were completed by March 2012, and T2K began once more to take data from that time. There was another interruption in 2013-2014 due to an accident in another beamline. Operations resumed in May, 2014, including the first anti-neutrino data, and we expect to continue to take data alternating muon neutrino and antineutrino beams for several years. By early 2015, the J-PARC neutrino beam was operating routinely at power levels above 300 kW.