Hot axions and the 𝐻0 tension

Scattering and decay processes of thermal bath particles involving heavy leptons can dump hot axions in the primordial plasma around the QCD phase transition. We compute their relic density, parameterized by an e ff ective number Delta N-eff of additional neutrinos. For couplings allowed by current bounds, production via scattering yields Delta N-eff less than or similar to 0.6 and Delta N-eff less than or similar to 0.2 for the cases of muon and tau, respectively. Flavor violating tau decays to a lighter lepton plus an axion give Delta N-eff less than or similar to 0.3. Such values of Delta N-eff can alleviate the tension between the direct local measurement of the Hubble constant H-0 and the inferred value from observations of the Cosmic Microwave Background, assuming Lambda CDM. We analyze present cosmological data from the Planck collaboration and baryon acoustic oscillations with priors given in terms of the axion-lepton couplings. For axions coupled to muons, the tension can be alleviated below the 3 sigma level. Future experiments will measure Delta N-eff with higher precision, providing an axion discovery channel and probing the role of hot axions in the H-0 tension.