Orateurs
Description
An ultra-strong magnetic field (B ≈ $\mathrm{10^{18}}$ Gauss) is anticipated during the early stages of heavy ion collisions. Such a strong magnetic field holds significant importance in QCD, including understanding topology of QCD vacuum, QCD phase transition, and nucleon structure. The directed flow or the first harmonic flow coefficient ($\mathrm{v}_1$), serves as a powerful tool not only for detecting the magnetic field but also for understanding its effects in the Quark-Gluon Plasma (QGP) medium (such as electrical conductivity). Additionally, $\mathrm{v}_1$ can capture information from the initial geometry of the system and also offer means to understand baryon transport. Recently, the STAR collaboration reported a substantial splitting of directed flow between positively and negatively charged identified particles in peripheral Au+Au and isobar (Ru+Ru and Zr+Zr) collisions. These results are consistent with the dominance of Faraday induction and Coulomb effect from the initial strong magnetic field [1].
In this presentation, we shall discuss the rapidity dependence of $\mathrm{v}_1$ and d$\mathrm{v}_1$/dy for $\mathrm{\pi}^{\pm}$, $\mathrm{K}^{\pm}$ and p ($\bar{\mathrm{p}}$) in Au+Au collisions at 7.7, 11.5, 14.6, and 19.6 GeV from Beam Energy Scan Phase-II, as well as in U+U collisions at 193 GeV measured by the STAR experiment. The $\mathrm{v}_1$ values will be reported as a function of transverse momentum, rapidity, and centrality. Additionally, the d$\mathrm{v}_1$/dy and the charge dependent difference, $\Delta d\mathrm{v}_1$/dy, of identified particles in U+U collisions will be compared to those in Au+Au and isobar (Ru+Ru and Zr+Zr) collisions. These findings will offer further insights into the initial electromagnetic field as well as baryon transport at various system sizes and beam energies.
[1]. STAR Collaboration, arXiv: 2304.03430
