Statistically-Anisotropic Tensor Bispectrum from Inflation
  We develop a possibility of generating tensor non-Gaussianity in a kind of
anisotropic inflation, where a $U(1)$ gauge field is kinetically coupled to a
spectator scalar field. Owing to this coupling, the coherent mode of the
electric field appears and softly breaks the isotropy of the Universe. We
compute the bispectrum of linearly-polarized tensor perturbations sourced by
the gauge field and find that it is strongly red-tilted and has distinctive
statistical anisotropies including higher-order multipole moments.
Interestingly, the tensor bispectra with the specific combinations of linear
polarization modes are dominant, and their amplitudes depend on the different
sets of multipole moments. This new type of statistically-anisotropic tensor
non-Gaussianity can be potentially testable with the upcoming cosmic microwave
background B-mode polarization experiments.
