  Quantum chromodynamics (QCD) contains field configurations which can be
characterized by a topological invariant, the winding number Q_w.
Configurations with nonzero Q_w break the charge-parity CP symmetry of QCD. We
consider a novel mechanism by which these configurations can separate charge in
the presence of a background magnetic field - the "Chiral Magnetic Effect". We
argue that sufficiently large magnetic fields are created in heavy ion
collisions so that the Chiral Magnetic Effect causes preferential emission of
charged particles along the direction of angular momentum. Since separation of
charge is CP-odd, any observation of the Chiral Magnetic Effect could provide a
clear demonstration of the topological nature of the QCD vacuum. We give an
estimate of the effect and conclude that it might be observed experimentally.
