TwInflation

The general structure of Hybrid Inflation remains a very well-motivated mechanism for lower-scale cosmic inflation in the face of improving constraints on the tensor-to-scalar ratio. However, as originally modeled, the waterfall field in this mechanism gives rise to a hierarchy problem (eta -problem) for the inflaton after demanding standard effective field theory (EFT) control. We modify the hybrid mechanism and incorporate a discrete twin symmetry, thereby yielding a viable, natural and EFT-controlled model of non-supersymmetric low-scale inflation, Twinflation. Analogously to Twin Higgs models, the discrete exchange-symmetry with a twin sector reduces quadratic sensitivity in the inflationary potential to ultra-violet physics, at the root of the hierarchy problem. The observed phase of inflation takes place on a hilltop-like potential but without fine-tuning of the initial inflaton position in field-space. We also show that all parameters of the model can take natural values, below any associated EFT-cutoff mass scales and field values, thus ensuring straightforward theoretical control. We discuss the basic phenomenological considerations and constraints, as well as possible future directions.

Automated summary

Hybrid Inflation is a well-motivated mechanism for cosmic inflation, but faces hierarchy problems; by introducing a discrete twin symmetry, these issues can be addressed, leading to a natural model of non-supersymmetric low-scale inflation.