  Relativistic heavy ion collisions provide the possibility to study the
topological charge in QCD matter through the event-by-event fluctuating net
axial charge or nonequal numbers of left- and right-handed quarks they generate
in the produced quark-gluon plasma. Based on the chiral kinetic approach for
nearly massless quarks and antiquarks in the strong vorticity field produced
along the normal direction of the reaction plane of non-central heavy ion
collisions, we show that a unique signal for the topological charge in QCD
matter can be identified from the asymmetric distribution of particles with
momenta pointing in the upper and lower hemispheres of the reaction plane as a
result of the fluctuating net axial charge.
