  Recent years, multiphoton pair production has become one of the most
promising approaches to investigate the Schwinger effect. However, the
production and evolution of chirality, a key topic in the study of this effect,
has not been thoroughly considered in the context of multiphoton pair
production. In this work, as the first step of filling this gap, we used the
Dirac-Heisenberg-Wigner formalism to study the production and evolution of
chirality in vacuum under the excitation of the spatially homogeneous electric
and magnetic fields $\mathbf{E}(t)$ and $\mathbf{B}(t)$ that satisfy
$\mathbf{E}(t)\parallel\mathbf{B}(t)$ and are only nonzero in a short time span
$0<t<\tau$, which serve as a simplified model of the laser beams in multiphoton
pair production experiments. Based on analytical calculation, we discovered
that, after the external fields vanish, an oscillation of pseudoscalar
condensate occurs in the system, which leads to the suppression of the
chirality of the produced fermion pairs; at the same time, it introduces a
special fermion energy $\epsilon_p=\sqrt{3} m$ at which the chiral charge
distribution of the fermions maximizes. This novel phenomenon could help us
identify different types of products in future multiphoton pair production
experiments.
