Tensor non-Gaussianities from Non-minimal Coupling to the Inflaton
  Tensor non-Gaussianity represents an important future probe of the physics of
inflation. Inspired by recent works, we elaborate further on the possibility of
significant primordial tensor non-Gaussianities sourced by extra fields during
inflation. Unitarity constraints limit the impact of extra (spinning) particle
content by means of a lower bound on the corresponding mass spectrum. For
spin-2 particles, this takes the form of the well-known Higuchi bound. Massive
($m\gtrsim H$) particles will typically decay during inflation unless they are
non-minimally coupled to the inflaton sector: the inflating field "lifts" the
dynamics of the extra field(s), effectively getting around the limits imposed
by unitarity. There exist several models that realize such a mechanism, but we
focus here on the set-up of [1] where, through an EFT approach, one is able to
capture the essential features common to an entire class of theories. In the
presence of an extra massive spin-2 particle, the interactions in the tensor
sector mimic very closely those in the scalar sector of quasi-single-field
inflationary models. We calculate the tensor bispectrum in different
configurations and extract its dependence on the extra tensor sound speed. We
show in detail how one may obtain significant tensor non-Gaussianities whose
shape-function interpolates between local and equilateral, depending on the
mass of the extra field. We also estimate the LISA response functions to a
tensor bispectrum supporting the intermediate-type shapes we find.
