Speaker
Description
We present the cosmology and observational probes of majoron dark matter in a high-scale seesaw framework with spontaneously broken lepton number. Right-handed neutrinos naturally generate light neutrino masses and can realize thermal leptogenesis, while the associated majoron is a light pseudoNambu-Goldstone boson that can be cosmologically stable and serve as a viable dark matter candidate for sub-MeV masses. We present the scenarios of both preinflationary and postinflationary histories of lepton number breaking. In the preinflationary scenario, majoron dark matter is produced by misalignment and constrained by CMB isocurvature. In the postinflationary scenario, the majoron abundance receives nonthermal contributions from spatially averaged misalignment, majoron radiation from global cosmic strings, and the collapse of the string-domain wall network, as well as a thermally produced component. This scenario can also be probed by future searches for the stochastic gravitational wave background produced by cosmic strings. We map the viable majoron dark matter parameter space and examine complementary probes from x-ray and soft γ-ray searches for majoron decays to photons, black hole superradiance, and Lyman-α forest observations. These results demonstrate that majoron dark matter offers a distinctive cosmological probe of high-scale lepton number breaking and thermal leptogenesis.