Charge-dependent directed flow in asymmetric nuclear collisions

TL;DR

This paper investigates how electric fields in asymmetric nuclear collisions influence the directed flow of charged hadrons, revealing insights into the electromagnetic response of early collision phases.

Contribution

It introduces a microscopic PHSD approach to study charge-dependent directed flow, highlighting the impact of early electromagnetic fields in asymmetric collisions.

Findings

Electric fields cause a splitting in directed flow for charged particles.

The flow splitting depends on pseudorapidity and transverse momentum.

Results provide insights into the electromagnetic properties of the early collision phase.

Abstract

The directed flow of identified hadrons is studied within the parton-hadron-string-dynamics (PHSD) approach for the asymmetric system Cu+Au in non-central collisions at $\sqrt{s_{NN}}$ = 200 GeV. It is emphasized that due to the difference in the number of protons of the colliding nuclei an electric field emerges which is directed from the heavy to the light nucleus. This strong electric field is only present for about 0.25 fm/c at $\sqrt{s_{NN}}$ = 200 GeV and leads to a splitting of the directed flow $v_1$ for particles with the same mass but opposite electric charges in case of an early presence of charged quarks and antiquarks. The microscopic calculations of the directed flow for $\pi^\pm, K^\pm, p$ and $\bar{p}$ are carried out in the PHSD by taking into account the electromagnetic field induced by the spectators as well as its influence on the hadronic and partonic quasiparticle trajectories. It is shown that the splitting of the directed flow as a function of pseudorapidity $\eta$ and in particular as a function of the transverse momentum $p_t$ provides a direct access to the electromagnetic response of the very early (nonequilibrium) phase of relativistic heavy-ion collisions and allows to shed light on the presence (and number) of electric charges in this phase.