Cosmology of an Axion-Like Majoron

Understand

We propose a singlet majoron model that defines an inverse seesaw mechanism in the $\nu$ sector.

• The majoron $\phi$ has a mass $m_\phi\approx 0.5$ eV and a coupling to the $\tau$ lepton similar to the one to neutrinos.
• In the early universe it is initially in thermal equilibrium, then it decouples at $T\approx 500$ GeV and contributes with just $\Delta N_{\rm eff}=0.026$ during BBN.
• At $T=26$ keV (final stages of BBN) a primordial magnetic field induces resonant $\gamma \leftrightarrow \phi$ oscillations that transfer 6% of the photon energy into majorons, implying $\Delta N_{\rm eff}=0.55$ and a 4.7% increase in the baryon to photon ratio.