Charge-dependent directed flow in Cu+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV

TL;DR

This study measures charge-dependent directed flow in Cu+Au collisions at 200 GeV, revealing a small but significant difference between positive and negative charges, supporting the influence of initial electric fields and suggesting early charge creation.

Contribution

First measurement of charge-dependent directed flow in Cu+Au collisions at 200 GeV, providing new insights into early electric field effects in heavy-ion collisions.

Findings

Finite charge-dependent flow difference observed

Results align qualitatively with electric field expectations

Difference smaller than PHSD model predictions

Abstract

We present the first measurement of charge-dependent directed flow in Cu+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics (PHSD) model, which suggests that most of the electric charges, i.e. quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, 1fm/$c$.