The effects of topological charge change in heavy ion collisions: “Event by event P and CP violation”

Quantum chromodynamics (QCD) contains field configurations which can be characterized by a topological invariant, the winding number Qw. Configurations with non-zero Qw 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.