The Majoron Cosmological Window: Dark Matter and Thermal Leptogenesis

Abstract: The majoron is the Nambu-Goldstone boson associated with the spontaneous breaking of a global $B-L$ symmetry. Remarkably, the minimal majoron framework can simultaneously address three key empirical indications of physics beyond the Standard Model: neutrino masses, the matter-antimatter asymmetry, and dark matter. In this work, we identify the cosmologically viable region in which majoron dark matter and high-scale thermal leptogenesis can be realised simultaneously. We show that successful leptogenesis plays a central role in making this scenario predictive: by constraining the right-handed-neutrino mass scale, it determines the irreducible freeze-in contribution to the majoron abundance and fixes the size of the couplings relevant for visible dark matter decays. Combining the irreducible dark matter production mechanisms with warm dark matter limits and indirect searches for decaying dark matter, we map the resulting majoron cosmological window and show that future X- and gamma-ray telescopes can probe part of the surviving parameter space.