We study the freeze-in scenario of leptogenesis via oscillations within the type-I seesaw model with two quasidegenerate heavy Majorana neutrinos $N_{1,2}$ having masses $M_2>M_1\sim (0.1–100)\mathrm{GeV}$, $(M_2-M_1)/M_1\ll 1$, focusing on the role of the $CP$-violation provided by the Dirac phase $\delta$ of the Pontecorvo-Maki-Nakagawa-Sakata lepton mixing matrix. We find that viable leptogenesis can be due solely to $CP$-violating values of $\delta$ and that the $N_{1,2}$ total mixing squared ${\mathrm{\Theta }}^2={\sum }_{\alpha }{\mathrm{\Theta }}_{\alpha }^2$ needed is within the reach of future experiments, ${\mathrm{\Theta }}_{\alpha }$ parametrizing the coupling to the charged lepton $\alpha =e$, $\mu$, $\tau$. Furthermore, the required parameter space differs from that associated with additional Casas-Ibarra sources of $CP$-violation. Future determination of $\delta$, ${\mathrm{\Theta }}^2$ and/or the ratios ${\mathrm{\Theta }}_{\tau }^2:{\mathrm{\Theta }}_{\mu }^2:{\mathrm{\Theta }}_e^2$ would provide a critical test of the considered scenario.

• Received 18 July 2023
• Accepted 13 October 2023

DOI:https://doi.org/10.1103/PhysRevD.108.L101302

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Published by the American Physical Society