Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
Implications on the electron fraction Ye (r-process nucleosynthesis)
As discussed in chapter 3, the electron fraction is a key parameter for the heavy
elements nucleosynthesis. Considering that electron and positron capture rates
are very small, the electron fraction can be written as
Y (0)
e =
1
1 + λp/λn
with the capture rates on p and n respectively given by
λp = σ¯νep(Eν)φ¯νe(Eν)dEν
and
λn =
σνen(Eν)φνe(Eν)dEν
(4.35)
(4.36)
(4.37)
where σνen and σ¯νep are the reaction cross sections for the corresponding processes
Eqs.(3.24-3.25). Using Eq.(4.34) for the electron neutrino flux φνe and the
electron anti-neutrino flux φ¯νe we realize that if neutrinos interact with matter at
the tree level, the CP-violating phase δ has no effect on Ye. Consequently, within
such assumptions δ has no influence on the heavy elements nucleosynthesis.
What if the fluxes of νµ and ντ are different?
Some differences in the muon/tau neutrino fluxes at emission can arise at the
level of the Standard Model, from example from radiative corrections to the
muon and tau neutrino cross sections [33]. On the other hand, if physics beyond
the Standard model operates during the infall and the shock-bounce stages of
the supernova evolution, mu and tau neutrino fluxes can differ and induce CPviolating
effects in the supernova environment. For example, generic neutrinoflavor
changing interactions can give rise to significant net mu and tau lepton
numbers [8]. In particular, if there are flavor changing interactions involving
charged leptons (e.g. a large scale conversion in the e − → µ − channel) one
could also end up with significantly different mu and tau neutrino fluxes. In
such cases one could have effects from the CP-violating phase on the electron
(anti)neutrino fluxes as well. After the completion of this work, we have found
a previous work by Akhmedov, Lunardini and Smirnov where a first analysis of
CP effects was performed [6]. Our findings are at variance with theirs. In fact,
the authors conclude that even if mu and tau neutrino fluxes are different, CPviolation
effects cannot be observed. Such a difference arises from the fact that
different initial conditions are taken in our calculations compared to those used in
Eq.(47) of [6], since we take flavour states and they take matter eigenstates. The
two only coincide with the infinite matter density limit. Indeed since the initial
neutrino states should be those at the neutrino-sphere, the neutrino conversion
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