Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
Phase effects
Figure (7.3) shows the electron anti-neutrino probability at the edge of the star,
at a time when the shock wave has reached the region where MSW conversion
might occur. To comprehend such behaviour, one can see the different resonance
regions for the different energies. The multiple resonance interferences induce
fast oscillations that show up as abrupt changes in the probabilities, below 20
MeV. Indeed, looking at Fig.(7.2), the top horizontal solid line corresponds to
the resonance density for a 20 MeV neutrino. Therefore, all neutrinos with energy
below 20 MeV would have a similar horizontal line above the 20 MeV line.
Those lines would cross the 2 s density profile in several points, so that such
neutrinos encounter multiple resonances. If these latter are semi-adiabatic, important
phases effects will appear as fast oscillations, this is what is observed on
Fig.(7.3). For energies above 45 MeV, an adiabatic conversion happened since
at that time (t =2 s) the forward shock didn’t reach yet the MSW conversion
for those energies as can be seen on Fig. (7.2) with the lower horizontal solid
line. The probability goes up to 0.6 and not 1 because with such oscillation parameters
the conversion is not complete. For energies between 20 and 45 MeV,
the forward shock has reached the MSW conversion regions, as can be seen on
Fig.(7.2). Therefore such energies have undergone non adiabatic resonances, the
electron anti neutrino survival probability stays at a near zero value.
7.2.1 Signal on Earth
Let us now consider a supernova explosion located at 10 kpc from Earth.
The ¯νe flux on Earth
In inverted hierarchy, for any value of θ13 (except zero), the electron (anti-
)neutrino probability becomes very small in presence of ν − ν interaction (see
chapter 5 Fig. 5.10). This implies that the corresponding fluxes have swapped
with the muon and tau neutrino fluxes and become “hot” at this point in their
propagation. This differs from the previously standard paradigm, where the electron
anti-neutrinos enter the region of the MSW resonance with a “cold” spectrum.
As we will see this fact will imply a specific time signal in core-collapse
supernova observatories. A second swapping of the (anti)neutrino fluxes may occur
when they reach the MSW resonance region, depending on the adiabaticity
of the resonance. One can see the consequences on the neutrino fluxes on Earth
in Fig.(7.4).
At the early times (t 1 s), for the inverted hierarchy and large θ13, antineutrinos
undergo an adiabatic MSW resonance and have a “cold” spectrum
on Earth 4 (Figure 7.4). When the shock wave passes through the MSW high
4 Note that the spectra mix slightly due to the θ12 rotation at the L resonance.
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