Gravitational waves induced by spinor fields
  In realistic model-building, spinor fields with various masses are present.
During inflation, spinor field may induce gravitational waves as a second order
effect. In this paper, we calculate the contribution of single massive spinor
field to the power spectrum of primordial gravitational wave by using retarded
Green propagator. We find that the correction is scale-invariant and of order
$H^4/M_P^4$ for arbitrary spinor mass $m_{\psi}$. Additionally, we also observe
that when $m_\psi \gtrsim H$, the dependence of correction on $m_\psi/H$ is
nontrivial.
