Spectator-induced electromagnetic effect on directed flow in heavy ion collisions

We estimate the electromagnetic effect of the spectator charge on azimuthal anisotropies observed in heavy ion collisions. For peripheral Pb+Pb reactions at the top energy of the CERN Super Proton Synchrotron, √sNN=17.3 GeV, we predict this effect to bring very large distortions to the observed directed flow, v₁, of positive and negative pions emitted close to beam rapidity. The overall magnitude of this effect is comparable to values of v₁ reported by the WA98 experiment in the vicinity of target rapidity. We argue that also at lower rapidities the spectator-induced electromagnetic effect may result in the splitting of values of v₁ observed for positive and negative pions. Such a splitting is visible in the data reported by the STAR Collaboration from the BNL Relativistic Heavy Ion Collider Beam Energy Scan. Both effects are sensitive to the space-time scenario assumed for pion emission. Therefore, they bring new information on the collision dynamics.