Gravitational wave trispectrum in the axion-SU(2) model
  We study the trispectrum of the gravitational waves (GWs) generated through
the dynamics of an axionic spectator field and SU(2) gauge fields during
inflation. In non-Abelian gauge theory, the gauge fields have four-point
self-interactions, which induce the tree-level GW trispectrum. We formulate
this type of the GW trispectrum including the non-dynamical contributions and
evaluate it in the equilateral limit as a unique signal of this model. We find
that the ratio of the GW trispectrum to the cube of the scalar power spectrum
can be as large as $\mathcal{O}(10^6)$ in the viable parameter space, which
could be captured in the CMB observations.
