19 July 2013

New Discovery from the T2K Experiment (7.5 sigma for Electron Neutrino Appearance)!

It has been pointed out to me that this has become more of a historical record than a blog....

If I were to blame it partly on a former student who has been promising to write an extensive article about his finishing his PhD on T2K and then going on to work on the LUX dark matter experiment in the States, that would be very poor form indeed, so I would not do such a thing!

Anyway, today is a big day for us on the T2K experiment (the photo above is the T2K Imperial group from a couple of years ago), and in particle physics as a whole. The early T2K observation from a couple of years back that muon neutrinos actually oscillate into electron neutrinos has become a fully-significant discovery (and is being presented to the European Physical Society meeting in Stockholm today by our friend and colleague Mike Wilking):

While the most obvious change is that the number of electron-neutrino candidate events has gone from 6 to 28, which is because of the longer period of time T2K has been running, and the higher intensity of the beam (plus of course the size of the underlying physics which causes the process), it is just as important that a lot of work has gone into understanding of the beam and the detectors and the ways neutrinos interact, to increase our certainty on the inputs that go into the physics statement that we can make.

That is to say that the likelihood that random fluctuations (which is to say just bad luck) could make us see what we see is less than one in a trillion. We are certain at that level that what we are seeing is the appearance of electron neutrinos in a muon neutrino beam, once they have travelled the 295 km from J-PARC to the Super-K detector! In the plot above, the green shows the electron neutrino candidates we'd expect to see if this new physics didn't happen. So much work goes into working out the size of the green histogram, so that we know that the actual electron neutrino candidates we see (in the black dots with uncertainties shown in the vertical bars) must mostly be caused by new physics, shown in pink.

We will still be able to make this measurement better, providing a high-precision value of the parameters that describe this in our current model of neutrino oscillations. This, combined with measurements made by other experiments of related but different processes, and other measurements by T2K itself, will continue to help us learn if our models do indeed describe the universe well, and what their implications are. That could be the topic of a future blog post—are there any finishing PhD students' who'd like to contribute an extensive article on this?

PS. This is the work of many many people, and here I quote the bit in our press release today that describes this:

The T2K experiment was constructed and is operated by an international collaboration. The current T2K collaboration consists of over 400 physicists from 59 institutions in 11 countries [Canada, France, Germany, Italy, Japan, Poland, Russia, Switzerland, Spain, UK and US]. The experiment is primarily supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). Additional support is provided by the following funding agencies from participating countries: NSERC, NRC and CFI, Canada; CEA and CNRS/IN2P3, France; DFG, Germany; INFN, Italy; Ministry of Science and Higher Education, Poland; RAS, RFBR and the Ministry of Education and Science of the Russian Federation; MICINN and CPAN, Spain; SNSF and SER, Switzerland; STFC, U.K.; DOE, U.S.A.

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