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.
