Baryon-Antibaryon Reconstruction at 50 MeV: Rest Masses and Cosmological Abundances

We reconstruct separate baryon and antibaryon equilibrium populations from fixed hadronic inventories, temperature, and a cosmological net-baryon-to-entropy constraint. An ideal hadron resonance gas is evaluated with PDG2014, PDG2020, and PDG2025 lists, retaining nominal masses and spin degeneracies. The baryon chemical potential is determined while mu_Q = mu_S = 0. At 50 MeV, PDG2025 yields 2.7261 × 10^30 baryons and 2.7172 × 10^30 antibaryons per cm³, with a net excess of 8.8765 × 10^27 cm⁻³. There are 306.1147 antibaryons per unit of net excess, and nucleons and antinucleons contribute 95.2176% of the total. The criterion n_antiB = n_B − n_antiB occurs at 38.203739 MeV, which rounds to the published reference of 38.2 MeV. Varying only the input eta = (6.12 ± 0.04) × 10^−10 gives 38.192814–38.214598 MeV. Comoving normalization relates these populations to an explicit present volume, while a rank test shows that the residual alone cannot identify a unique pair population or history. All 24 numerical checks and five archived CSV outputs were reproduced; a further momentum-integral calculation checks aggregate densities and entropy at two states. The baseline leaves nonzero net charge and strangeness. It provides a reproducible equilibrium reference; chemical-neutrality controls and kinetic evolution belong to the companion extension. Numerical agreement does not determine total physical uncertainty.