Cosmology of an axion-like majoron

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IFAE Hybrid Seminar


by Prof. Manel Masip (Granada U.)


We propose a singlet majoron model that defines an inverse seesaw mechanism in the neutrino sector. The majoron has a mass around 0.5 eV and axion-like couplings with interesting cosmological implications. In particular, right after BBN a primordial magnetic field may induce resonant gamma-majoron oscillations that transfer 6% of the CMB energy into majorons, implying ΔNeff=0.55 and a 4.7% increase in the baryon to photon ratio. Near recombination the majorons enter in thermal contact with the neutrinos and finally they decay, setting a final value of Neff=3.85. This boost in the expansion rate relaxes the Hubble tension (we obtain H0=(71.4±0.5) km/s/Mpc), while the processes nub nu <-> phi suppress the free streaming of these particles and make the model consistent with large scale structure observations. Its lifetime and the fact that it decays into neutrinos instead of photons lets this axion-like majoron avoid the strong bounds that affect  other axion-like particles of similar mass and coupling to photons.


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