Speaker
Description
We derive updated cosmological constraints on light axion-like particles (ALPs) coupled to leptons and photons using a full phase-space treatment of their production in the primordial plasma. The resulting non-thermal phase-space distributions are consistently propagated into calculations of cosmological observables, allowing a precise determination of their impact on $\Delta N_{\rm eff}$. Combining latest available CMB data with BBN measurements, we obtain $95\%$ credible limits of $f_a > 1.63 \times 10^6\,\mathrm{GeV}$, $9.41 \times 10^6\,\mathrm{GeV}$, and $8.06 \times 10^4\,\mathrm{GeV}$ for couplings to electrons, muons, and taus, respectively, and $g_{a\gamma} < 1.98 \times 10^{-8}\,\mathrm{GeV}^{-1}$ for photon couplings. Compared to astrophysical and laboratory limits, these cosmological bounds are particularly competitive for the ALP couplings to $\mu$ and $\tau$. Inclusion of baryon acoustic oscillation data mildly relaxes these bounds. We also present forecasts for future CMB experiments, discussing the importance of an exact phase-space treatment for robust constraints on ALP interactions.