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).
Taken from http://t2k-experiment.org/t2k/