The current theory of particle physics, known as the Standard Model, includes 12 particles which make up matter (the physical contents of the Universe). Of these 6 are quarks: they combine to form particles such as protons and neutrons, which in turn make up the atomic nuclei found in the periodic table. Then there are the 3 leptons: a family containing the electron, which amongst other things is responsible for electricity, and two identical but heavier particles known as the muon (μ) and tau (τ). Together, leptons and quarks form all the matter which we see around us.
The last 3 particles in the Standard Model are not often encountered in every day life: the neutrinos.
Although they are the least understood particles in the Standard Model, neutrinos are the most numerous particles in the Universe: around 50 trillion neutrinos from the Sun pass through your body every second, and many more come from elsewhere. Why then do we not notice them?
Neutrinos can only interact with other matter via the "Weak nuclear" force, the most feeble of forces in the Standard Model. Such weak interactions mean that the probability that they will interact with nearby particles is extremely low. So rare are their interactions, that the average neutrino could pass through over 10 lightyears of lead without ever noticing.
You may be wondering how, if neutrinos hardly interact with matter, we can see them in particle physics experiments.
The three neutrinos are the electron neutrino (νe), muon neutrino (νμ) and tau neutrino (ντ). You may notice that they are named after the three leptons mentioned earlier. This is because, in the rare event that a neutrino does interact with matter it will produce the lepton with which it is associated. For example, if an electron neutrino were to interact with a proton, an electron would be produced.
This is how we tell that neutrinos are travelling through our detector: we look not for the neutrinos but for the particles resulting from their interactions, and we can identify which type of neutrino interacted by identifying the particle produced. Of course, we need a lot of neutrinos if we hope to see even a few interactions!
Neutrinos were first postulated by Wolfgang Pauli in 1930 and first detected in 1956 by Reines and Cowan (a feat for which they received the 1995 Nobel Prize). However it was in the late 1960's that they became the focus of serious research. Theorists modelling how nuclear fusion worked in the Sun had made very specific predictions about the number of electron neutrinos being produced, but experiments designed to measure those neutrinos consistently found around a third of the number they were expecting. Likewise, measurements of muon neutrinos, from cosmic ray collisions in the upper atmosphere, also showed a reduction in the number compared with that predicted.
Eventually, experiments were able to explain these mysterious disappearances in terms of "neutrino oscillations" - the oscillation from one neutrino type to another. The electron neutrinos from the Sun were oscillating into muon and tau neutrinos (which the solar experiments could not measure), and atmospheric muon neutrinos were oscillating into tau neutrinos. Further experiments measured this phenomenon more accurately using better detectors and by generating neutrinos artificially in neutrino beams.
|νe → νμ , ντ
|νμ → ντ
|νμ → νe
The next step is to look for oscillations from muon neutrinos to electron neutrinos. Measurement of this behaviour would not only complete our picture of how oscillations take place but would also make it possible to investigate how neutrinos can contribute to one of the biggest mysteries in physics: the matter - anti-matter asymmetry of the Universe.
T2K is the first of the "next generation" neutrino oscillation experiments designed to look for this transition: it features a high powered muon neutrino beam with detectors placed off the beam axis to give a cleaner energy spectrum. It will for the first time measure muon to electron neutrino oscillations while also improving the precision of previously measured paramaters.
The experiment begins with the generation a beam of muon neutrinos at the J-PARC accelerator complex on the Eastern coast of Japan. Immediately downstream is the near-detector (ND280) which will enable a measurement of the initial beam. Construction of ND280 and analysis of its data is the primary contribution of the UK in T2K. The beam then travels almost 300km through Japan during which time it is hoped that some of the muon neutrinos will oscillate into electron neutrinos, the number of which will be measured by Super-Kamiokande (Super-K).
Super-K is a cylindrical tank, over 40m tall, which contains 50kT of ultra-pure water. It is located 1000m underground in a disused mine and can identify both electron and muon neutrino interactions within the water. The experiment will then compare the data from SuperK with that in ND280 to measure, at high precision, neutrino oscillation paramaters which have never been observed before!
T2K began taking data early in 2010 with early results expected in Summer 2011.