Making the Universe at 20 MeV

We present a testable mechanism of low-scale baryogenesis and dark matter production in which neither the baryon nor lepton number are violated. Charged D mesons are produced out of equilibrium at tens of MeV temperatures. The D mesons quickly undergo CP-violating decays to charged pions, which then decay into dark-sector leptons without violating the lepton number. To transfer this lepton asymmetry to the baryon asymmetry, the dark leptons scatter on additional dark-sector states charged under the lepton and baryon number. Amusingly, this transfer proceeds without electroweak sphalerons, which are no longer active at such low scales. We present two example models which can achieve this transfer while remaining consistent with current limits. The required amount of CP violation in charged D meson decays, while currently allowed, will be probed by colliders. Additionally, the relevant decays of charged pions to dark-sector leptons have been constrained by the PIENU and Paul Scherrer Institute experiments and will be further explored in upcoming experiments.

Automated summary

This paper presents a testable mechanism for low-scale baryogenesis and dark matter production without violating baryon or lepton numbers. It discusses the production of charged D mesons and their CP-violating decays to charged pions, which then decay into dark-sector leptons. The transfer of lepton asymmetry to baryon asymmetry is achieved through scattering of dark leptons on additional dark-sector states, without the involvement of electroweak sphalerons at low scales. The models presented in the paper are consistent with current limits and the required CP violation in charged D meson decays will be probed by colliders.