New Bounds on Heavy QCD Axions from Big Bang Nucleosynthesis
This research uses Big Bang Nucleosynthesis (BBN) to set stringent new limits on heavy QCD axions by tracking their effect on neutron-to-proton ratios during the early universe. The analysis incorporates complex hadronic physics, including kaon contributions and secondary hadron tracing, yielding a robust upper bound of $ au_a ext{lesssim } 0.02$ seconds for masses above $0.3 ext{ GeV}$, surpassing current CMB constraints.