Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
2.3 The unknown third mixing angle: θ13
Knowing the mixing angle θ13 is of first importance for many reasons, in addition
to a deeper knowledge of the MNSP matrix. As it has been said earlier, the CPviolating
phase is directly linked to sinθ13, the smaller θ13 is, the more difficult
it will be to measure δ. Moreover, θ13 plays a great role in the core-collapse
supernova environment. In fact, what occurs at resonance at high density depends
on its value on the one hand and can influence the nucleosynthesis on the other
hand as we will see in the following chapters.
2.3.1 Upper bound for θ13
Since there is no powerful ντ sources available on Earth, neutrino experimentalists
have looked for the disappearance of the electron anti-neutrinos produced
by nuclear reactors. The reason why people are looking at anti-neutrinos is simple:
nuclear fission reactions produce, via beta-decay, low energy (2-10 MeV)
anti-neutrinos with a high precision on the intense emitted flux. Measuring the
remaining anti-neutrino flux at a certain distance will give information on the
oscillation parameters.
Currently, the most stringent constraint on the third mixing angle is given by the
CHOOZ experiment. CHOOZ is a little town in France near the France-Belgium
border where two nuclear reactors of 1450 MW each are present. The CHOOZ
experiment is an underground short-baseline reactor-neutrino vacuum-oscillation
experiment which detects electron anti-neutrino by a liquid scintillator calorimeter
located at a distance of about 1 Km from the source. The limit obtained
at 90% level of confidence is: sin 2 2θ13 < 0.2 for ∆m 2 23 = 2.0 × 10−3 eV 2 which
corresponds to θ13 9 ◦ . Improving the upper bound of θ13 is the goal of several
starting or future experiments. Though not everyone is agreeing on that, some
models accounts for a value of θ13 close to the current one. There is also indication
on the 1 σ level from the combined analysis of experimental results. If it
turns out to be true, it will be extremely exciting for the difficult search of the
CP-violating phase δ.
2.4 The hierarchy problem
Currently experimental results show that only three flavours of neutrinos are
existing since the LSND results [16, 17] were invalidated by the Mini-Boone experiment
in 2007 [34]. As we said before, the flavour basis is associated to the
mass basis composed of three different neutrino masses.
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