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- contours in the - e plane for BBN (N = 3) and the adopted D and 4He abundances (Barger et al. 2003b).