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4.3. Neutrino Asymmetry (xie neq 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 (xie), which regulates the pre-BBN neutron-to-proton ratio through the reactions in Equation 5. In Figure 15 are shown the 1- and 2-sigma contours in the eta - xie plane for BBN (for Nnu = 3) and the adopted abundances of D and 4He. As seen before for Delta Nnu neq 0, while a fit to the data can be achieved for xie neq 0, the data are not inconsistent with xie = 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 Delta Nnu and xie are free to differ from zero, see Barger et al. (2003b).

Figure 15

Figure 15. The 1-, 2-, and 3-sigma contours in the eta - xie plane for BBN (Nnu = 3) and the adopted D and 4He abundances (Barger et al. 2003b).

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