Leptogenesis and Low Reheating Temperatures

We study leptogenesis during non-instantaneous reheating in the canonical type-I seesaw framework, with the dominant source of right-handed neutrino (RHN) production being non-thermal from inflaton decays ($\phi \rightarrow NN$). While matter-like reheating ($w_\phi=0$) fails to be compatible with standard leptogenesis for very low reheating temperatures, the situation is strikingly different for generalized Starobinsky potentials approximated by $V(\phi)\propto\phi^k$ with $k\geq4$ about the minimum. In the latter cases, the observed baryon asymmetry can readily be obtained for arbitrarily low reheating temperatures above the BBN bound of $\sim4$ MeV. We study radiation-like reheating ($w_\phi=1/3$, $k=4$) in detail, showing that the evolving effective mass of the inflaton condensate leads to kinematic shutoff of the $\phi\rightarrow NN$ channel, which qualitatively changes the leptogenesis dynamics. We include a detailed treatment of the effects of fragmentation of the inflaton condensate. The final baryon asymmetry depends primarily on only two parameters: the inflaton-RHN coupling, $y_{\phi NN}$, and the CP-violating parameter $|\epsilon|$. Interestingly, the final asymmetry is largely insensitive to the RHN mass, the reheating temperature, and the RHN decay rate. While we focus on fermionic reheating, we show that the general features of these results also hold for bosonic reheating to scalars.