Nucleosynthesis constraints on the faint vector portal

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New Abelian U(1)' gauge bosons Vμ can couple to the Standard Model through mixing of the associated field strength tensor Vμν with the one from hypercharge, Fμνʸ. Here we consider early Universe sensitivity to this vector portal and show that the effective mixing parameter with the photon, κ, is being probed for vector masses in the GeV ballpark down to values 10⁻¹⁰ κ 10⁻¹⁴ where no terrestrial probes exist. The ensuing constraints are based on a detailed calculation of the vector relic abundance and an in-depth analysis of relevant nucleosynthesis processes.