The T2K experiment is a “long-baseline” neutrino experiment where neutrinos are produced in an accelerator north of Tokyo and then shot across Japan to the Super-Kamiokande detector underneath the Japanese Alps.  Super-Kamiokande is a giant underground water Cherenkov experiment, designed to capture neutrinos from the Sun and sky: the 11,000 inner detector photomultiplier tubes (PMTs) record photons from the charged products of neutrino interactions in the ultra-pure water.  In the T2K experiment, Super-K acts at the far target of the neutrino beam.

In 1998, Super-K showed that muon neutrinos produced by cosmic ray collisions in the Earth’s atmosphere “disappear” by changing to almost-invisible tau flavor: the neutrinos “oscillate” from one flavor to another by interference of mass states.  Such flavor change is only possible if neutrinos have mass.  Neutrino masses and the parameters which govern neutrino flavor oscillation are deeply connected to both fundamental particle physics and cosmology. Over the next few years, the Super-K atmospheric neutrino result was confirmed by other experiments.  The beam neutrinos “went missing” in exactly the numbers expected, and with exactly the expected energy dependence predicted by the oscillation hypothesis.

The next physics quest for Super-K is the search for  the unknown neutrino oscillation parameter, “Theta _13” as part of the T2K experiment.  The signature of non-zero Theta_13 is a tiny amount of electron neutrino appearance in a beam of muon neutrinos.  The T2K experiment is designed to measure this parameter by looking for muon neutrinos produced in an accelerator at the JPARC center north of Tokyo to transform into electron neutrinos after they travel 295 km across Japan.  Since this is a very small effect, a powerful beam is needed to create just a few of these events.

In the latest T2K results, the accelerator made a beam pulse of neutrinos over two million times.   Six events consistent with an electron neutrino were observed, although only 1.5 events would have been seen if muon neutrinos don’t oscillate into electron neutrinos.  The probability that the observation is just due to a chance fluctuation is less than 1%.