Magnetohydrodynamics, charged currents, and directed flow in heavy ion collisions

High-energy heavy-ion collisions at RHIC and LHC produce the strongest magnetic fields on Earth. So far there has been no explicit measurement of them. Gursoy et al. provide a quantitative estimate of the space-time evolution of these fields in an expanding medium, and suggest a method to infer them from experimental data on charged-particle correlations. This work provides a major step toward understanding the effect of the magnetic field on directed flow and should be a benchmark for future work.