Charge splitting of directed flow and charge-dependent effects in pion spectra in heavy ion collisions

TL;DR

This paper investigates how electromagnetic fields in heavy ion collisions cause charge-dependent distortions in pion spectra and flow, revealing insights into the space-time evolution of pion production.

Contribution

It introduces a detailed analysis of charge splitting effects in pion spectra and flow, linking them to the spatial and temporal dynamics of pion emission.

Findings

Charge splitting of directed flow varies with rapidity.

Distortion of charged pion spectra depends on emission site distance from spectators.

Distance $d_E$ decreases with increasing pion rapidity, below 1 fm near beam rapidity.

Abstract

The large and rapidly varying electric and magnetic fields induced by the spectator systems moving at ultrarelativistic velocities induce a charge splitting of directed flow, $v_1$, of positive and negative pions in the final state of the heavy ion collision. The same effect results in a very sizeable distortion of charged pion spectra as well as ratios of charged pions ($\pi^+/\pi^-$) emitted at high values of rapidity. Both phenomena are sensitive to the actual distance between the pion emission site and the spectator system. This distance $d_E$ appears to decrease with increasing rapidity of the pion, and comes below $\sim$1~fm for pions emitted close to beam rapidity. In this paper we discuss how these findings can shed new light on the space-time evolution of pion production as a function of rapidity, and on the longitudinal evolution of the system created in heavy ion collisions.