Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
Chapter 8
CP-violation effects on the
neutrino degeneracy parameter
There are presently three observational evidences for the Big Bang Model: the
universal expansion, the Cosmic Microwave Background Radiation (CMBR), and
the Big Bang Nucleosynthesis (BBN) (also called primordial nucleosynthesis).
The latter evidence provides a probe of the Universe during its early evolution
[107]. Indeed, it is during this epoch that the primordial abundances of light
nuclides (D, 3 He, 4 He, and 7 Li) were produced. They are sensitive to the universal
density of baryons and to the early Universe expansion rate, which at early times
is governed by the energy density of relativistic particles such as photons and
neutrinos.
At a certain time, just before BBN, neutrinos decouple from matter and will
conserve their thermal spectrum with a proper rescaling of the parameters due to
redshift as the Universe will expand. Such relic neutrino background has never
been directly detected so we must resort to indirect means to infer its properties.
One of the most useful tools available is naturally BBN [111]. By putting neutrino
oscillations together with BBN, we may shed light on neutrino properties and/or
cosmology. Our aim in this chapter is to investigate the influence of the CPviolating
phase, contained in the MNSP matrix, on the value of the electron
neutrino lepton asymmetry.
8.1 Introduction
8.1.1 The neutrino degeneracy parameter and implications
In the very early Universe, baryon-antibaryon pairs were as abundant as radiation.
As the Universe expands and cools, the pairs annihilate, leaving behind
any baryon excess established during the earlier evolution of the Universe. Subse-
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