Phenomenology of Majorons

Majorons are the Goldstone bosons associated to lepton number and thus closely connected to Majorana neutrino masses. Couplings to charged fermions arise at one-loop level, including lepton-flavor-violating ones that lead to decays $\ell\to \ell' J$, whereas a coupling to photons is generated at two loops. The typically small couplings make massive majorons a prime candidate for long-lived dark matter. Its signature decay into two mono-energetic neutrinos is potentially detectable for majoron masses above MeV.