  It is proposed to identify a strong electric field - created during
relativistic collisions of asymmetric nuclei - via the observation of
pseudorapidity and transverse momentum distributions of hadrons with the same
mass but opposite charge. The results of detailed calculations within the
Parton-Hadron String Dynamics (PHSD) approach for the charge-dependent directed
flow $v_1$ are presented for semi-central Cu+Au collision at
$\sqrt{s_{NN}}=200$ GeV incorporating the inverse Landau-Pomeranchuk-Migdal
(iLPM) effect, which accounts for a delay in the electromagnetic interaction
with the charged degree of freedom. Including the iLPM effect we achieve a
reasonable agreement of the PHSD results for the charge splitting in $v_1(p_T)$
in line with the recent measurements of the STAR Collaboration for Cu+Au
collisions at $\sqrt{s_{NN}}=200$ GeV while an instant appearance and coupling
of electric charges at the hard collision vertex overestimates the splitting by
about a factor of 10. We predict that the iLPM effect should practically
disappear at energies of $\sqrt{s_{NN}} \approx$9 GeV, which should lead to a
significantly larger charge splitting of $v_1$ at the future FAIR/NICA
facilities.
