Abstract: The study of correlation functions of density perturbations could shed light on the interactions of the inflaton with other particles during the epoch of primordial inflation. In particular, the detection of an oscillating pattern in the bispectrum would imply the presence of heavy particles at a scale close to the Hubble rate during inflation.
A minimal scenario providing this signature only requires a coupling of SM particles to the inflaton. We focus on the fermions, whose dispersion relation can be modified by the coupling to the inflaton, leading to an enhanced particle production even if their mass during inflation is larger than the Hubble scale.
We study two particularly interesting cases. If the Higgs potential displayed a second minimum at high energies, where the Higgs field resided during inflation before going back to the electroweak vacuum during the thermal phase, we could detect the simultaneous contributions of two SM fermions to the bispectrum. Such a discovery would be a tremendous step towards understanding the vacuum instability of the Higgs potential, and could have fascinating implications for anthropic considerations.
Another remarkable possibility is that the Higgs vacuum during inflation is dynamically set by the contribution of the fermion density to the Higgs potential. This case is characterised by a unique relation between amplitude and frequency of the oscillating pattern: this one-parameter model offers a very minimal and predictive signature detectable in cosmological collider physics.