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.
