Spectator induced electromagnetic effect on directed flow in heavy ion collisions

TL;DR

This paper predicts that spectator-induced electromagnetic effects significantly distort the directed flow of pions in heavy ion collisions, providing new insights into collision dynamics and pion emission timing.

Contribution

It introduces the first quantitative estimate of electromagnetic distortions on directed flow in heavy ion collisions, linking these effects to collision dynamics and pion emission scenarios.

Findings

Electromagnetic effects cause large distortions in directed flow $v_1$.

Splitting of $v_1$ for positive and negative pions observed in data.

Effects are comparable to experimental measurements at CERN and RHIC.

Abstract

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, $\sqrt{s_{NN}}=17.3$ GeV, we predict this effect to bring very large distortions to the observed directed flow, $v_1$, of positive and negative pions emitted close to beam rapidity. The overall magnitude of this effect is comparable to values of $v_1$ reported by the WA98 experiment. We argue that also at lower rapidities, the spectator induced electromagnetic effect may result in the splitting of values of $v_1$ observed for positive and negative pions. Such a splitting is visible in the data reported by the STAR Collaboration from the RHIC 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.