Fermion production during and after axion inflation
  We study derivatively coupled fermions in axion-driven inflation,
specifically $m_\phi^2\phi^2$ and monodromy inflation, and calculate particle
production during the inflationary epoch and the post-inflationary axion
oscillations. During inflation, the rolling axion acts as an effective chemical
potential for helicity which biases the gravitational production of one fermion
helicity over the other. This mechanism allows for efficient gravitational
production of heavy fermion states that would otherwise be highly suppressed.
Following inflation, the axion oscillates and fermions with both helicities are
produced as the effective frequency of the fermion field changes
non-adiabatically. For certain values of the fermion mass and axion-fermion
coupling strength, the two helicity states are produced asymmetrically,
resulting in unequal number-densities of left- and right-helicity fermions.
