Resonantly-Enhanced Baryogenesis through Asymmetric Capture by Primordial Black Holes

Abstract

Primordial black holes (PBHs) can generate the Baryon Asymmetry of the Universe (BAU) through asymmetric capture of baryon-charge-carrying particles, but previous realizations of this mechanism typically yield an asymmetry well below the observed value because the relevant CP-violating scattering asymmetry is loop suppressed. We show that this limitation can be overcome through resonant CP violation in a mixed system of two nearly degenerate unstable fermions, $Y_{1,2}$, whose mass splitting is comparable to their decay widths. Rare production of the coherent $Y_1$--$Y_2$ state, followed by CP-asymmetric return and loss decays, generates an $\mathcal{O}(0.1)$ difference in integrated decay probabilities while preserving the global baryon charge at the Lagrangian level. Embedding this resonant source in the PBH-capture transport system yields the observed baryon asymmetry. We first reproduce the mechanism in a zero-temperature, sudden-evaporation treatment, which gives ${\eta }_B^{T=0}\simeq 1.74\times 10^{-10}\simeq 2.00{\eta }_B^{\mathrm{obs}}$. We find that after including thermal corrections and continuous PBH evaporation and entropy production, the resulting BAU becomes ${\eta }_B=8.72\times 10^{-11}$ in agreement with the observed value. An interesting finding of the model is that even if PBH absorption remains active, we would still have BAU ${\eta }_B^{\text{no evap}}\sim 9.1\times 10^{-12}\sim 0.105{\eta }_B^{\mathrm{obs}}$. We further investigate gravitational-wave signatures associated with the primordial fluctuations responsible for PBH formation. The corresponding scalar-induced gravitational-wave spectra peak at $f\sim 0.16\text{–}0.17\mathrm{MHz}$, with $h^2{\Omega }_{\mathrm{GW}}^{\mathrm{peak}}\sim (3\text{–}7)\times 10^{-8}$. Under idealized sensitivity assumptions, U-DECIGO can achieve signal-to-noise ratios above 10 for the benchmark spectra.

arXiv comment28 pages, 9 figures, 5 tables