Cosmological signatures of neutrino seesaw mechanism

The tiny neutrino masses are most naturally explained by seesaw mechanism through singlet right-handed neutrinos, which can further explain the matter-antimatter asymmetry in the Universe. In this work, we propose a new approach to study cosmological signatures of neutrino seesaw through the interaction between inflaton and right-handed neutrinos, which respects the shift symmetry. In our framework, after inflation the inflaton predominantly decays into right-handed neutrinos and its decay rate is modulated by the fluctuations of the Higgs field, which act as the source of curvature perturbations. This gives a new realization of Higgs modulated reheating, and it produces primordial non-Gaussian signatures that can be measured by the forthcoming large-scale structure surveys. We demonstrate, for the first time, that the measurements of non-Gaussianity by these surveys can probe the neutrino seesaw scale and have the potential to constrain a large portion of the seesaw parameter space, opening up a new window for directly testing the high-scale seesaw mechanism.