Illustration: Carin Cain after Ref. [3]

Figure 1: When two nuclei collide with a nonzero impact parameter, their velocity vectors define a reaction plane (grey). The magnetic field $B$ created by the moving nuclei points, on average, in the direction of the relative angular momentum $L$ of the nuclei, perpendicular to the reaction plane. The presence of the magnetic field leads to a local violation of P and CP symmetry for strongly interacting, electrically charged particles (quarks). Because of this chiral magnetic effect, particles carrying charges of the same sign will be preferentially emitted into the same hemisphere (arrows $1$ and $2$); those with charges of opposite sign will be emitted into different hemispheres (arrows $1$ and $2′$). Fluctuations of the charge symmetry with respect to the collision plane, which have been observed by the STAR Collaboration, may therefore be a signature of local parity violation.