Transport Meeting

Recently, the STAR collaboration has presented preliminary results on the splitting of $v_1(y)$-slope ($\Delta (\mathrm{d} v_1/\mathrm{d}y)$) between positively and negatively charged hadrons as a function of centrality. The observed sign change of $\Delta (\mathrm{d} v_1/\mathrm{d}y)$ with centrality is proposed as a signature of the electromagnetic field. But, using a suitable Glauber-based initial condition of energy and baryon distribution that captures the rapidity dependence of directed flow of identified hadrons, we demonstrate that the measured sign change can be described with our model of baryon stopping without considering any electromagnetic field effects. This suggests that baryon stopping in the initial stage serves as a large background to such signals of the electromagnetic field. However, we have found that the measurement of the centrality-dependent splitting of directed flow ($\Delta v_1$) between protons and antiprotons by the STAR collaboration is extremely responsive to changes in the baryon diffusion coefficient ($\kappa_{\mathrm{B}}$) of the strongly interacting medium. Hence, we are able to provide the first estimate of $\kappa_{\mathrm{B}}$ by model-to-data comparison. Furthermore, we propose that the measurement of transverse momentum differential $\Delta v_1$ can put additional constraints on $\kappa_{\mathrm{B}}$.