Making the Universe at 20 MeV

TL;DR

This paper proposes a low-scale baryogenesis and dark matter production mechanism involving CP-violating decays of charged D mesons at around 20 MeV, which transfer lepton asymmetry to baryons without violating lepton number.

Contribution

It introduces a novel low-energy baryogenesis model utilizing meson decays and dark-sector interactions, avoiding electroweak sphalerons and remaining testable with current experiments.

Findings

The mechanism can generate the observed baryon asymmetry at low temperatures.

Current experimental constraints are compatible with the proposed models.

Upcoming collider and decay experiments will test key aspects of this scenario.

Abstract

We present a testable mechanism of low-scale baryogenesis and dark matter production in which neither 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 lepton number. To transfer this lepton asymmetry to the baryon asymmetry, the dark leptons scatter on additional dark-sector states charged under 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 Amusingly, this transfer proceeds without electroweak sphalerons, which are no longer active at such low scales.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 PSI experiments and will be further explored in upcoming experiments.