素粒子論セミナーのお知らせ

Heavy QCD axions are compelling candidates for new physics: they solve the strong CP problem and can be experimentally accessible if their mass lies in the GeV range. When their lifetime exceeds a microsecond, cosmology becomes the unique probe. In particular, Big Bang Nucleosynthesis (BBN) is highly sensitive to the hadronic decay products of heavy axions. By carefully analyzing the dynamics of neutron decoupling and the observed 4He abundance, we derive the strongest existing bounds on axion lifetime, reaching as short as 0.02 seconds. We also find that the K^0_L contribution has not been correctly accounted for in the past literature and actually dominates in relevant regimes. Therefore, this result is relevant to BBN bounds on other hypothetical particles, such as sterile neutrinos.