A new study explores the possibility of generating the observed baryon asymmetry in the universe through leptogenesis, even with very low cosmic reheating temperatures, close to the Big Bang Nucleosynthesis (BBN) bound of approximately 4 MeV. Traditionally, leptogenesis requires significantly higher reheating temperatures. This work focuses on the canonical type-I seesaw framework, where the dominant production of right-handed neutrinos (RHN) is non-thermal, originating from inflaton decays (φ → NN).

The research reveals that, while matter-like reheating (with an equation of state parameter w_φ=0) is incompatible with standard leptogenesis at very low temperatures, the situation changes drastically for generalized Starobinsky potentials, approximated by V(φ)∝φ^k with k≥4. In these scenarios, the observed baryon asymmetry can be readily obtained. A particular case studied in detail is radiation-like reheating (w_φ=1/3, k=4), where the evolving effective mass of the inflaton condensate leads to a kinematic shutoff of the φ → NN channel, qualitatively altering the leptogenesis dynamics.

The authors include a detailed treatment of the effects of inflaton condensate fragmentation. Interestingly, the final baryon asymmetry primarily depends on only two parameters: the inflaton-RHN coupling (y_φNN) and the CP-violating parameter (|ε|). A key finding is that the final asymmetry is largely insensitive to the RHN mass, the reheating temperature, and the RHN decay rate. Although the study focuses on fermionic reheating, it is shown that the general features of these results also hold for bosonic reheating to scalars.