Observation of the electromagnetic field effect via charge-dependent directed flow in heavy-ion collisions at the Relativistic Heavy Ion Collider

The STAR collaboration
Phys.Rev.X 14 (2024) 011028, 2024.

Abstract (data abstract)
Non-central collisions between ultra-relativistic heavy nuclei can produce strong magnetic fields on the order of $10^{18}$ Gauss, and the evolution of the electromagnetic field could leave an imprint on the final-state particles. In particular, particles and anti-particles with opposite charges will receive opposite contributions to their rapidity-odd directed flow, $v_1(\mathsf{y})$. Here we present the charge-dependent measurements of $dv_1/d\mathsf{y}$ near midrapidity for $\pi^{\pm}$, $K^{\pm}$, and $p(\bar{p})$ in Au+Au and isobar ($_{44}^{96}$Ru+$_{44}^{96}$Ru and $_{40}^{96}$Zr+$_{40}^{96}$Zr) collisions at $\sqrt{s_{NN}}=$ 200 GeV, and in Au+Au at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the $v_1$ signal on collision system, particle species, and collision centrality can be qualitatively and semi-quantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the $u$ and $d$ quarks transported from initial-state nuclei, those in peripheral events reveal the contributions from the Faraday induction and Coulomb effect for the first time in heavy-ion collisions.

  • Figure 5a

    10.17182/hepdata.139915.v1/t1

    Directed flow of $p$ and $\bar{p}$ vs rapidity in Au+Au 200 GeV 50-80% centrality.

  • Figure 5b

    10.17182/hepdata.139915.v1/t2

    Directed flow of $p$ and $\bar{p}$ vs rapidity in Zr+Zr and Ru+Ru 200 GeV (combined) 50-80% centrality.

  • Figure 5c

    10.17182/hepdata.139915.v1/t3

    Directed flow of $p$ and $\bar{p}$ vs rapidity in Au+Au 27 GeV 50-80% centrality.

  • Figure 5d

    10.17182/hepdata.139915.v1/t4

    \Delta v_1 between proton and anti-proton vs rapidity in Au+Au 200 GeV 50-80% centrality.

  • Figure 5e

    10.17182/hepdata.139915.v1/t5

    \Delta v_1 between proton and anti-proton vs rapidity in Zr+Zr and Ru+Ru (combined) 200 GeV 50-80% centrality.

  • Figure 5f

    10.17182/hepdata.139915.v1/t6

    \Delta v_1 between proton and anti-proton vs rapidity in Au+Au 27 GeV 50-80% centrality.

  • Figure 6a

    10.17182/hepdata.139915.v1/t7

    \Delta dv1/dy as a function of centrality in Au+Au 200 GeV.

  • Figure 6b

    10.17182/hepdata.139915.v1/t8

    \Delta dv1/dy as a function of centrality in Ru+Ru and Zr+Zr 200 GeV

  • Figure 6c

    10.17182/hepdata.139915.v1/t9

    \Delta dv1/dy as a function of centrality in Au+Au 27 GeV.

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