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 1018 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, v1(y). Here we present the charge-dependent measurements of dv1/dy near midrapidity for π±, K±, and p(p¯) in Au+Au and isobar (4496Ru+4496Ru and 4096Zr+4096Zr) collisions at sNN= 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 v1 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 p¯ vs rapidity in Au+Au 200 GeV 50-80% centrality.

  • Figure 5b

    10.17182/hepdata.139915.v1/t2

    Directed flow of p and 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 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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