Large lepton asymmetry for small baryon asymmetry and warm dark matter

We propose a resonant leptogenesis scenario in a $U(1{)}_{B-L}$ gauge extension of the standard model to generate large lepton asymmetries for cosmological baryon asymmetry and dark matter. After the $B-L$ number is spontaneously broken, the inflaton can pick up a small vacuum expectation value for the mass splits of three pairs of quasidegenerately heavy Majorana neutrinos and the masses of three sterile neutrinos. With thermal mass effects of sphalerons, the observed small baryon asymmetry can be converted from large lepton asymmetries of individual flavors, although total lepton asymmetry is assumed zero. The mixing between sterile and active neutrinos is elegantly suppressed by the heavy Majorana neutrinos. Before the active neutrinos start their strong flavor conversions, the sterile neutrinos as warm dark matter can be produced by resonant active-sterile neutrino oscillations to reconcile x-ray and Lyman-$\alpha$ bounds. Small neutrino masses are naturally realized by seesaw contributions from the heavy Majorana neutrinos and the sterile neutrinos.