Anomaly-free flavor models for Nambu–Goldstone bosons and the 3.5keV X-ray line signal

We pursue a possibility that a pseudo-Nambu–Goldstone boson is lurking around or below the intermediate scale. To this end we consider an anomaly-free global flavor symmetry, and construct models where the pseudo-Nambu–Goldstone boson is coupled preferentially to leptons. The experimental and astrophysical bounds derived from couplings to photons and nucleons are significantly relaxed. If sufficiently light, the pseudo-Nambu–Goldstone boson contributes to dark matter, and interestingly, it generally decays into photons through couplings arising from threshold corrections. We show that the recent hint for the X-ray line at about 3.5keV can be explained by the decay of such pseudo-Nambu–Goldstone boson of mass about 7 keV with the decay constant of order 1010GeV, if the electron is charged under the flavor symmetry.