Speaker
Description
We investigate lepton-flavour-conserving (LFC) and lepton-flavour-violating (LFV) axion-lepton couplings using cosmological data, addressing the specific mass ranges where the standard $\Delta N_\mathrm{eff}$ approximation fails. We implement a full phase-space treatment of thermal axion production and incorporate finite axion mass effects to test these couplings against Planck 2018 CMB and DESI DR2 BAO measurements. Our analysis significantly strengthens previous cosmological bounds in the mass range $0.1~\mathrm{eV} \lesssim m_a \lesssim 1~\mathrm{keV}$. For $m_a \sim 10$ eV, cosmological limits on LFC electron and muon couplings, as well as LFV $\mu$-$e$ couplings, improve by a factor of about 5 compared to standard $\Delta N_\mathrm{eff}$ limits, while bounds on couplings involving the tau lepton improve by several orders of magnitude. Crucially, for LFV tau couplings, our bounds surpass recent Belle-II limits on $\tau \to \ell a$ decays for $m_a \gtrsim 0.3$ eV. We also apply approximate Ly-$\alpha$ forest constraints, extending the exclusion reach up to $10^8$ GeV (LFC) and $10^9$ GeV (LFV) for masses approaching 1 keV. Applied to the QCD axion, our analysis excludes parameter space with $m_a \gtrsim 2$ eV for $C_\tau = 1$, otherwise allowed by $\Delta N_\mathrm{eff}$ alone. Based on arXiv:2511.14864.