Cosmological Collider Signatures from Right-Handed Neutrino Loop

Abstract

We study cosmological collider (CC) signatures generated by right-handed neutrino loops in the setup of inflation combined with neutrino seesaw mechanism. We formulate the inflaton interaction with the right-handed neutrino through a unique dimension-5 operator respecting shift symmetry, which induces an effective chemical potential in the slow-roll background, leading to helicity-dependent right-handed-neutrino production and enhancing the CC signatures. The right-handed neutrino is described by two-component Weyl spinor with Majorana mass term. Using the Schwinger-Keldysh (SK) formalism, we derive a set of seed integrals for fermion propagators of the right-handed Majorana neutrino. With these we compute the factorized nonlocal contributions to the three-point inflaton correlator generated by the right-handed-neutrino triangle loop. We show that the chemical potential can substantially soften the heavy-mass Boltzmann suppression and amplify the oscillatory non-Gaussianity signatures associated with the dominant helicity mode. These provide a systematic framework for analyzing the fermion loop signatures in the cosmological collider physics and demonstrate that the heavy right-handed neutrinos associated with seesaw mechanism may leave observable imprints in the primordial non-Gaussianities.