Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
are not exactly thermal but are pinched7 . One way to parametrize the pinched
neutrino spectra is to introduce an effective temperature Tα and an effective degeneracy
parameter ηνi (which has the same sign for neutrinos and antineutrinos
and cannot be considered as the chemical potential) in the Fermi-Dirac thermal
spectrum for each species α:
Lνα(r, Eν) = 1
4πr 2
L0 να
T 3 να 〈Eνα〉F2(η)
E2 να
1 + exp (Eνα/Tνα − ηνα)
(3.22)
where F2(η) is the Fermi integral, L 0 να and Tνα are the luminosity and temperature
at the neutrino sphere. The “non-electron” neutrinos (νµ, ντ, ¯νµ, ¯ντ) have the
same neutral current interactions inside the supernova, and their original fluxes
are expected to be approximately equal 8 . In what follows we will neglect the
difference of fluxes. Since we suppose Fermi-Dirac distribution, the temperature
hierarchy can be derived from the average energies Typically,
Tνe ≈ 3 − 4 MeV , T¯νe ≈ 5 − 6 MeV , Tνx ≈ 7 − 9 MeV . (3.23)
For a pinched spectrum, ηi > 0. The value of ηi is the same for all νx species
(neutrinos as well as antineutrinos, since they have the same interactions), and
are in general different from ηνe or η¯νe. Note that their value, which covers the
range 0 − 5 is model dependent.
3.2.3 R-process nucleosynthesis
One of the major open issues in nuclear astrophysics is to identify the site for the
heavy elements nucelosynthesis (heavier than iron). Such a process occur thanks
to rapid neutron captures and is called the r-process nucleosynthesis [23, 35].
The most probable site appears at present to be the late stages of core-collapse
supernova explosion (about 1s after bounce) outside the proton neutron star
and in presence of a strong neutrino wind. However none of present simulations
are able to produce the observed abundances. In fact, neutrino (anti-neutrino)
interactions with n (p) reduce the neutron available flux significantly and kills
the r-process [119, 112]. This fact is currently under serious investigation. Any
effect that can potentially modify the neutron to proton ratio by a few percent
might solve this crucial problem. Several possible solutions have been evoked so
far, but for the moment this puzzle remains unsolved. In this thesis, we explore
the possibility that a non-zero CP-phase affects the neutron to proton ratio which
might be a solution of this hot open issue.
7Note that other parametrizations like power-law have been shown to nicely fit the simulations.
8The presence of real muons in the central part of the star leads to a nonzero chemical
potential of the muon neutrinos and hence to a difference of fluxes [75]. However, in the
neutrino-sphere with T ≈ 6 − 8 MeV, the concentration of muons is smaller than 1%. The
presence of one-loops correction can also modify differently the νµ fluxes from the ντ fluxes.
This fact will be used in the next chapter in relation with CP-violation.
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