At the 2012 Neutrino conference in Kyoto, Japan, the T2K collaboration presented new results on electron neutrino appearance from muon neutrino that confirmed their previous published results in PRL, in July 2011, which reported the first single experimental indication that θ13 is non-zero and large with a 2.5 σ level of significance.
Based on the data collected until May 15, 2012, corresponding to 2.56×1020 POT, 10 “electron neutrino appearance” candidate events were observed in the T2K far detector, Super-Kamiokande. This observation further solidifies the previous result. We now find that the probability of background fluctuation to yield 10 or more events (p-value) is 0.08% corresponding to a 3.2 σ level of significance, making it extremely unlikely that a statistical fluctuation is at the origin of the observed events.
Analysis of this data using three different methods yielded consistent results with the fitted central value of 0.104 (+0.060 -0.045) for sin22θ13 assuming δ=0, Δm223=2.4×10-3 eV2, θ23 = π/4, and normal mass hierarchy.
Recent precision results on θ13 from nuclear power reactor experiments are in agreement with this T2K result. While the reactor experiments are based on the disappearance of electron anti-neutrinos, the T2K result shows the appearance of electron neutrinos in a muon neutrino beam, which is also sensitive to the potential CP-violation effects of Nature. The consistency of these two very different measurements of θ13 gives us confidence in our understanding of the underlying physics, and provides us with a unique opportunity to resolve the unknown neutrino mass hierarchy. It also opens the door to the search for CP violation (which would result in oscillations being different for neutrinos and anti-neutrinos), which might be a hint to the physics behind the excess of matter over anti-matter in the universe.