Seminarium "Teoria cząstek elementarnych i kosmologia"

Axions appearing in extensions of the Standard Model can be produced thermally in the early universe through their couplings to leptons, leaving imprints on cosmological observables. Most existing analyses rely on the ΔN_eff approximation, which assumes ultra-relativistic axions and breaks down for axion masses above ~0.1 eV. I will present new model-independent constraints on both lepton-flavour-conserving and lepton-flavour-violating axion-lepton couplings, derived from Planck 2018 CMB and DESI DR2 BAO data, using the full phase-space Boltzmann equation for thermal axion production and including finite axion mass effects. The resulting bounds are significantly stronger than those from ΔN_eff across the mass range from 0.1 eV to 1 keV, with the most dramatic improvement for couplings involving the tau lepton, where our cosmological bounds surpass the most recent Belle-II limits on τ → ℓa decays. The results have direct implications for the QCD axion.