Parity Violating Non-Gaussianity from Axion-Gauge Field Dynamics
  We study scalar-tensor-tensor and tensor-scalar-scalar three point cross
correlations generated by the dynamics of a transiently rolling spectator
axion-$\mathrm U(1)$ gauge field model during inflation. In this framework,
tensor and scalar fluctuations are sourced by gauge fields at the non-linear
level due to gravitational interactions, providing a chiral background of
gravitational waves while keeping the level of scalar fluctuations at the
observationally viable levels at CMB scales. We show that the gravitational
couplings between the observable sector and gauge fields can also mediate
strong correlations between scalar and tensor fluctuations, generating an
amplitude for the mixed type three-point functions that is parametrically
larger -- $f_{\rm NL} \simeq \mathcal{O}(1-10) (r/0.01)^{3/2}$-- compared to
the single-field realizations of inflation. As the amplification of the gauge
field sources are localized around the time of horizon exit, the resulting
mixed bispectra are peaked {\it close} to the equilateral configurations. The
shape dependence, along with the scale dependence and the parity violating
nature of the mixed bispectra can serve as a distinguishing feature of the
underlying axion-gauge field dynamics and suggest a careful investigation of
their signatures on the CMB observables including cross correlations between
temperature T and E,B polarization modes.
