Parity violation in stochastic gravitational wave background from
  inflation
  We study the inflationary implications of a novel parity-violating gravity
model, which modifies the teleparallel equivalent of general relativity by
introducing the Nieh-Yan term coupled to an axion-like field. The
parity-violating Nieh-Yan term results in the velocity birefringence of
gravitational waves (GWs) and triggers the tachyonic instability only for one
of the two circular polarization states. We consider that the inflaton is
identified as the coupled axion-like field with a wiggly potential
characterized by steep cliffs connected by smooth plateaus. During inflation,
the temporary fast roll of axion on the cliff-like region leads to the
significant enhancement of the tensor perturbations in one polarization state
with the wave numbers that exit the horizon around this period. In this setup,
the resulting energy spectrum for GWs presents a sizable localized bump
involving the contribution of only one polarization state. This chiral GW
background is detectable by LISA and Taiji, and its chirality can be determined
by correlating two detectors, which provide an opportunity to probe the
inflation and test the gravity model.
