Improved cosmological constraints on axion-lepton interactions

TL;DR

This paper uses cosmological data to set new, stronger limits on axion-lepton interactions, especially for axion masses above 0.1 eV, surpassing collider constraints and impacting QCD axion and ALP models.

Contribution

It provides updated cosmological bounds on axion-lepton couplings, incorporating finite axion mass effects, and demonstrates these bounds are stronger than previous collider and astrophysical constraints.

Findings

Bounds on lepton-flavor-conserving axion-tau coupling improved by orders of magnitude.

Lower bounds on axion decay constants exceed 10^6 and 10^8 GeV for masses above 1 eV.

Cosmological constraints surpass collider limits for axion masses above 0.3 eV.

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 $\Delta N_{\rm eff}$ 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 $\tau \rightarrow la$ decays, where $l=e$ or $\mu$. The collider constraints on $\mu \rightarrow ea$ 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).