Large tensor non-Gaussianity from axion-gauge field dynamics

We show that an inflation model in which a spectator axion field is coupled to an $SU(2)$ gauge field produces a large three-point function (bispectrum) of primordial gravitational waves, Bₕ, on the scales relevant to the cosmic microwave background experiments. The amplitude of the bispectrum at the equilateral configuration is characterized by Bₕ/Pₕ²=O(10)×ΩA^-1, where ΩA is a fraction of the energy density in the gauge field and Pₕ is the power spectrum of gravitational waves produced by the gauge field.