Charge-Dependent Directed Flow in $\mathrm{Cu}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

We present the first measurement of charge-dependent directed flow in $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. 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 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, $1\mathrm{fm}/c$.

Figure 1

Example of a noncentral $\mathrm{Cu}+\mathrm{Au}$ collision viewed in the transverse plane showing an initial electric field $\overrightarrow{E}$ caused by the charge difference between two nuclei. ${\mathrm{\Psi }}_1^{\mathrm{Au}\text{−}\mathrm{SP}}$ denotes the direction of Au spectators.

Figure 2

Directed flow of positive and negative particles from minimum bias $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, as a function of $p_T$, in five centrality bins. The difference between the positive and negative spectra is shown in the lower panels, where the open boxes show the systematic uncertainties. See the text for the definition of the positive direction for $v_1$.

Figure 3

Directed flow of positive and negative particles and the difference between the two spectra as a function of $p_T$ in 10%–40% centrality in $\mathrm{Cu}+\mathrm{Au}$ and $\mathrm{Au}+\mathrm{Au}$ collisions. The PHSD model calculations [15] for charged pions with and without the initial electric field (EF) in the same centrality region are presented for comparison. Note that the charge difference of $v_1$ with the EF on is scaled by 0.1.

Figure 4

Directed flow of positive and negative particles and the difference between the two spectra as a function of $\eta$ in the 10%–40% centrality bin, where a positive $\eta$ denotes the Cu-going direction.