T2K at Lancaster University
Lancaster University is involved in the T2K (Tokai to Kamioka) neutrino experiment in Japan which is studying neutrino oscillations. A future phase of the experiment might investigate the reasons why equal amounts of matter and antimatter are not seen in the Universe.
The Physics of Neutrinos
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. They are produced in large numbers in the Sun, and solar neutrinos can pass all the way through the Earth without interacting.
According to quantum physics, all particles (including neutrinos) also have a wavelike nature. A neutrino travels as a combination of three waves, and these waves must be added together to determine the flavour of the neutrino. If neutrinos have different masses, the relative phases (i.e. the relative positions of the crests and troughs) of the three waves change as they travel. After travelling some distance, the relative phases can change sufficiently that the three waves combine to produce a neutrino of a different flavour. As the three waves continue to travel, the relative phases can then revert to their original configuration, and they combine to produce the original neutrino flavour.
These flavour changes follow a sine wave pattern, and, for this reason, are known as "neutrino oscillations". The oscillations depend on the energy of the neutrinos: low-energy neutrinos oscillate in a shorter distance than high-energy neutrinos.
The standard model of particle physics used to assume that neutrinos are massless. However results from the Japanese SuperKamiokande detector in 1998 and from the SNO experiment in 2001 showed that neutrinos definitely do oscillate. This can only happen if they have different masses, which means that at least two of them have non-zero masses. These masses have not yet been measured, but an upper limit of 0.3 eV (one millionth of the mass of an electron) has been set for the sum of the three masses by astrophysical observations of large-scale structures in the Universe.
About the T2K Experiment
The T2K experiment sends an intense beam of muon neutrinos across Japan from J-PARC, Tokai (on the east coast) to SuperKamiokande, Kamioka (in western Japan), a distance of 295 km. The 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 three main components: a proton accelerator at J-PARC that produces the neutrino beam, the near detectors (INGRID and ND280) which measure the numbers of neutrinos at J-PARC before they can change flavours, and the far detector, SuperKamiokande. The measurements at the near detectors are used to predict the number of muon neutrinos that would be seen in SuperKamiokande if there were no oscillations.
At the SuperK detector, the neutrinos pass through a very large tank of ultra-pure water. Charged particles produced by interactions between neutrinos and the water cause Cerenkov light to be emitted in SuperK. This light is detected by photomultiplier tubes, each one sensitive enough to detect a single photon.
Having stated earlier that neutrinos can pass through the Earth without interacting, it may seem difficult to believe that they will interact with a large tank of water. However the T2K neutrinos have much higher energies than solar neutrinos, and high-energy neutrinos are more likely to interact. Despite this, the majority of T2K neutrinos pass through SuperK without interacting; a very intense beam and a large volume of water are required to produce a relatively small number of interactions.
T2K is searching for oscillations from muon neutrinos to electron neutrinos. These oscillations have never been observed, and, if T2K were to find them, it would represent a major discovery. If they are found, T2K will, in a subsequent phase, study differences between matter and antimatter by comparing antineutrino oscillations with neutrino oscillations.
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 (the actual measurement is of the difference between the squares of these masses). T2K can make more accurate measurements than previously possible since the detectors are placed off-axis with respect to the centre of the neutrino beam.
The world's first off-axis neutrino detector
T2K is the world's first off-axis neutrino experiment, with ND280 and SuperK set at a 2.5 degree angle to 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 flavour by the time they reach SuperK. 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 oscillations from muon to tau neutrinos and the neutrino mass difference than those from previous experiments.
Lancaster University and T2K
Lancaster University physicists and technicians working on the T2K project have helped to build the ND280 near detector, which measures the number of muon neutrinos in the beam for comparison with those at SuperKamiokande.
The UK built the Electromagnetic Calorimeter (ECal). Lancaster built the DSECal, the most downstream part of the Ecal, where some of the first neutrino events were detected. The DSECal was a working prototype upon which the procedures for all 13 ECal modules were tested.
Lancaster physicists and technicians played a unique role in devising the procedures, and then contributed strongly to calibrating and commissioning the detector.
Events in SuperK, the T2K far detector
Neutrinos cannot be detected directly in SuperK, but only indirectly through the products of their interactions with the water. Many interactions of muon neutrinos produce muons and pions, while interactions of electron neutrinos often produce electrons. These muons, pions and electrons travel faster than the velocity of light in water (which is three-quarters of its velocity in a vacuum).
Charged particles such as muons, electrons and charged pions 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, this light is emitted as a cone known as Cerenkov radiation. This cone of light is detected by the photomultipliers in SuperK as a ring.
The image shows the first-ever T2K event observed at SuperK: in this event, an uncharged pion has decayed to two photons, and each photon has produced an electron-positron pair. Electrons and positrons are charged, and each pair has produced one of the rings visible in the image.