Speaker
Description
Heavy-ion collisions can produce an ultra-strong magnetic field, the evolution of which was predicted to decrease (increase) the directed flow slope, $dv_1/dy$, for positively (negatively) charged particles [1, 2]. In this work, we study this effect with large statistics datasets accumulated for Au+Au, $^{96}_{44}$Ru+$^{96}_{44}$Ru, and $^{96}_{40}$Zr+$^{96}_{40}$Zr isobar collisions at $\sqrt{s_{NN}}=$ 200 GeV, and Au+Au collisions at $\sqrt{s_{NN}}=$ 27 GeV. The charge dependent $dv_1/dy$ splitting, $\Delta(dv_1/dy)$, will be presented for $\pi^{\pm}$, $K^{\pm}$, and (anti)proton. A finite $\Delta(dv_1/dy)$ between protons and anti-protons has been observed and it changes from positive to negative as a function of centrality from central to peripheral collisions. This is the first observation of a significant negative $\Delta(dv_1/dy)$ between proton and anti-protons. A similar decreasing trend of slope difference between $K^+$ and $K^-$ has also been observed in Au+Au collisions at $\sqrt{s_{NN}}=$ 200 GeV and 27 GeV, and in isobar collisions with less significance. The slope difference between $\pi^+$ and $\pi^-$ is negative and decreases as a function of centrality in Au+Au collisions at $\sqrt{s_{NN}}=$ 27 GeV, while no significant slope difference is observed in Au+Au and isobar collisions at $\sqrt{s_{NN}}=$ 200 GeV. Our measurements of significant negative $\Delta(dv_1/dy)$ cannot be explained by conventional mechanisms (e.g. transported quarks), but qualitatively agree with the theoretical prediction with an ultra-strong electromagnetic field in peripheral collisions.
[1] U. Gursoy, D. Kharzeev, E. Marcus $et~al.$, Phys. Rev. C ${\bf 98}$, 055201 (2018).
[2] U. Gursoy, D. Kharzeev and K. Rajagopal, Phys. Rev. C $\bf{89}$, 054905 (2014).
