Improved cosmological constraints on axion-lepton interactions

TL;DR

This work delivers model-independent cosmological constraints on axion–lepton interactions by solving the full momentum-dependent Boltzmann equation for thermally produced axions and fitting to Planck CMB plus DESI DR2 BAO data. By treating both LFC and LFV couplings and explicitly incorporating axion mass, the analysis finds that mass effects significantly strengthen bounds for m_a ≳ 0.1 eV, with some channels (notably axion–tau and axion–muon) becoming the leading constraints in the 0.3 eV–keV range. Using an extended ΛCDM framework, the study shows cosmology can outperform current collider and some astrophysical limits, and it provides complementary warm DM constraints from Ly-α that further limit the viable axion parameter space. The results are relevant for QCD axions and ALPs, highlighting regions where cosmology offers the strongest suppression of axion couplings and where future surveys could further tighten these bounds.

Abstract

We present updated cosmological constraints on axion-lepton interactions based on state-of-the-art computations of the thermal axion abundance. By combining Planck Cosmic Microwave Background (CMB) data with baryon acoustic oscillation (BAO) measurements from DESI DR2, we derive improved limits on both lepton-flavor-conserving (LFC) and lepton-flavor-violating (LFV) axion couplings. Incorporating finite axion mass effects substantially strengthens the bounds for axion masses above 0.1 eV compared to those inferred from the $ΔN_{\rm eff}$ constraint alone. The bounds on the LFC axion-tau coupling and LFV axion couplings to tau and muon or electron are improved by several orders of magnitude and the lower bound on the axion decay constant may exceed $10^6$ and $10^8$ GeV, respectively, for axion masses above 1 eV. Our cosmological constraints on LFC axion couplings to muons and taus and LFV axion couplings to tau and muon or electron are stronger than all other constraints for masses above 0.3 eV. In particular, they are stronger than recent collider constraints from Belle-II on $τ\rightarrow la$ decays, where $l=e$ or $μ$. The collider constraints on $μ\rightarrow ea$ decays are weaker than the cosmological constraints for axion masses above 100 eV. Our results are relevant for both the QCD axion and axion-like particles (ALPs).

Figures (10)

• Figure 1: $\Delta N_{\rm eff}$ computed via fBE as a function of $f/|C|$ for all considered lepton channels. Solid curves correspond to lepton-flavor conserving (LFC) processes, while dashed curves correspond to lepton-flavor violating (LFV) processes. The results for electron interactions are plotted with an orange curve (only LFC), muon interactions -- with purple curves, and tau interactions -- with golden curves. Dashed grey horizontal lines mark the current constraint from Planck and the predicted sensitivity of Simons Observatory. See text for more details.
• Figure 2: Summary of constraints on LFC axion couplings $f/|C_l|$ as a function of axion mass $m_a$. The colored points connected with solid lines indicate the 95% C.L lower bounds on the couplings derived from CMB and BAO measurements (Planck 2018 + DESI data). The dashed lines of the same color represent the mass-independent bound on $f/|C_l|$ from the simplified $\Delta N_\mathrm{eff}$ analysis. The grey region is excluded by the approximate wDM constraint (see Sec. \ref{['Sec: wDM']}). Green shading denotes mass-independent astrophysical limits. The dotted lines indicate the cross-sections of the parameter space for the QCD axions with the values of $\log_{10} |C_l|= [-3, -2, -1, 0, 1]$, from right to left ($|C_l|=1$ is highlighted by a darker color). See the text for more details.
• Figure 3: Same as Fig. \ref{['fig:lfc_mass_limits_scatter_plot']}, but for LFV axion couplings.
• Figure 4: Constraints on the axion decay constant $f/|C|$ as a function of the axion mass $m_a$ from the axion contribution to the present-day DM abundance, $\omega_{\rm dm}=0.12$. Each panel corresponds to a distinct axion-lepton interaction with LFC (LFV) interactions shown on the left (right) panels. Solid lines correspond to the calculation of the axion abundance via fBE approach, while the dashed lines correspond to the nBE approach. We show two contours of the present-day axion energy density: $\omega_a = \omega_{\rm dm}$ (CDM bound) and $\omega_a = 0.1\omega_{\rm dm}$ (wDM bound). The shaded region below the $\omega_a = 0.1\omega_{\rm dm}$ indicates the parameter space that we estimate to be excluded due to overproduction.
• Figure 5: Summary of 95% C.L. bounds on $f/|C|$ for LFC and LFV axion-lepton couplings. The orange bars represent the weakest cosmological limits based on the upper bound on $\Delta N_{\mathrm{eff}} < 0.3\,$ from Planck. Blue bars show the bounds for the axion decay constant for a QCD axion with $C=1$, while green bars illustrate two representative scenarios with non-vanishing axion mass, $m_a=10$ eV and $m_a=0.3$ eV. The strongest non-cosmological constraints are represented by the dark–grey bars and correspond to astrophysical (collider) bounds for LFC (LFV) couplings.