  When the Standard Model Higgs sector is extended with a complex singlet that
breaks global lepton number symmetry spontaneously, a massless Goldstone boson
called the Majoron J arises. In addition to increasing Higgs invisible decay
through mixing, the Majoron can generally have flavor-changing interactions
with fermions. We use the type-III seesaw mechanism to demonstrate the
existence of such couplings with both charged leptons and neutrinos. This opens
up new channels to search for the Majoron. We use the experimental data such as
muonium-anti-muonium oscillation and flavor-changing neutrino and charged
lepton decays to put constraints on the couplings. Besides, we propose a
polarization asymmetry of flavor-changing l to l' J decays that can reveal the
chiral information of the interactions.
