More viable parameter space for leptogenesis

Lepton flavor asymmetries generated at the onset of the oscillations of sterile neutrinos with masses above the electroweak scale can be large enough to partly survive washout and to explain the baryon asymmetry of the Universe. This opens up new regions of parameter space, where leptogenesis is viable within the type-I seesaw framework. In particular, we find it possible that the sterile neutrino masses are substantially below ${10}^9\mathrm{GeV}$, while not being degenerate. However, the required reheat temperature that is determined by the beginning of the oscillations lies some orders of magnitude above the sterile neutrino mass scale.

Figure 1

Final BAU normalized to the observed value for the reference parameters.

Figure 2

Ratio of the final BAU assuming either full equilibration (solid) or no equilibration of ${\mathrm{R}}_e$ compared to the result assuming partial equilibration for the reference parameters. The dotted line for the ratio one should help to guide the eye and to demonstrate that both limiting cases apply for small and large $M_1$.

Figure 3

The temperature $T_{\text{osc}}$ (solid line) and the temperature $T_{\text{res}}$ (dashed line) for the reference parameters. The dotted lines represent $T_{\mathrm{W}e,\mu ,\tau }$ from the bottom right to top left.

Figure 4

Fraction of the initial asymmetry that remains after washout for the reference parameters. Result based on the approximations described in Sec. 2 (solid), where the asymmetries in $q_{Ni}$ and ${\mathrm{\Delta }}_{\mu ,\tau }$ are neglected are compared with the results from the equations of the Appendix (dashed), where the detailed evolution of the individual flavors are taken into account.

Figure 5

Washout parameters $\sum _i{K}_{ie,\mu }$ for a seesaw model with three sterile neutrinos in the mass ratio $1:2:3$ for $M_1:M_2:M_3$ and the light neutrino mass $m_1=2.5\mathrm{meV}$, over a flat random distribution for the angles and phases $\delta$, ${\alpha }_1$, ${\alpha }_2$, ${\omega }_{23}$, ${\omega }_{13}$, and ${\omega }_{12}$ (dark blue). The same for a model with two sterile neutrinos (light magenta, implemented here by setting $m_1=0$, ${\omega }_{23}=0$, and ${\omega }_{13}=\pi /2$).

Figure 6

Evolution of various combinations of flavored asymmetries for the reference parameters and with $M_1=2\times {10}^4\mathrm{GeV}$ after the initial oscillations at $z\approx z_{\text{osc}}$: ${\mathrm{\Delta }}_e$ (solid line), ${\mathrm{\Delta }}_{\mu }+{\mathrm{\Delta }}_{\tau }$ (dashed line), ${\mathrm{\Delta }}_{\text{gen}}$ (dot-dashed line).