4.3. Neutrino Asymmetry
(
e
0)
The tension between D and 4He can also be relieved by
nonstandard neutrino physics (see
Fig. 3). Although the asymmetry
(difference between the numbers of particles and antiparticles) in
charged leptons, tied to that in the baryons by charge neutrality
of the Universe, must be very small, the neutrino asymmetry is
unconstrained observationally. Of relevance to BBN is the asymmetry
between the electron neutrinos and the electron antineutrinos
(e),
which regulates the pre-BBN neutron-to-proton ratio through the reactions
in Equation 5. In Figure 15 are shown
the 1- and 2-
contours
in the 
-
e
plane for BBN (for
N
= 3) and
the adopted abundances of D and 4He. As seen before for
N
0, while a fit to the data
can be achieved for
e
0, the data are not
inconsistent with
e =
0. Furthermore,
as is shown in Figure 15, BBN constrains the
allowed range for neutrino asymmetry to be very small. For further
implications for neutrino physics and for a discussion of the case
where both
N and
e are
free to differ from zero, see
Barger et al. (2003b).
 |
Figure 15. The 1-, 2-, and
3- |