  We investigate the phenomenon of electron-positron pair production from
vacuum in the presence of a strong electric field of circular polarization. By
means of a nonperturbative approach based on the quantum kinetic equations
(QKEs), we numerically calculate helicity-resolved momentum distributions of
the particles produced and analyze the corresponding helicity asymmetry. It is
demonstrated that the external rotating field tends to generate left-handed and
right-handed particles traveling in opposite directions. Generic symmetry
properties of the momentum spectra are examined analytically by means of the
QKEs and also confirmed and illustrated by direct numerical computations. The
helicity signatures revealed in our study are expected to provide a firmer
basis for possible experimental investigations of the fundamental phenomenon of
vacuum pair production in strong fields.
