Interplay of drag by hot matter and electromagnetic force on the directed flow of heavy quarks

Abstract

Rapidity-odd directed flow in heavy ion collisions can originate from two very distinct sources in the collision dynamics i. an initial tilt of the fireball in the reaction plane that generates directed flow of the constituents independent of their charges, and ii. the Lorentz force due to the strong primordial electromagnetic field that drives the flow in opposite directions for constituents carrying unlike sign charges. We study the directed flow of open charm mesons D 0 and D 0 in the presence of both these sources of directed flow. The drag from the tilted matter dominates over the Lorentz force resulting in same sign flow for both D 0 and D 0 , albeit of different magnitudes. Their average directed flow is about ten times larger than their difference. This charge splitting in the directed flow is a sensitive probe of the electrical conductivity of the produced medium. We further study their beam energy dependence; while the average directed flow shows a decreasing trend, the charge splitting remains flat from √ sNN = 60 GeV to 5 TeV.

Electromagnetic fields and directed flow in large and small colliding systems at ultrarelativistic energies

“…In panel (a) the solid red line correspond to hydrodynamic simulations with the inclusion of the electromagnetic fields ("Hydro+EM") from Refs. [23,35] and the dashed brown line is the results of transport calculations with the AMPT model [36]. In panel (b) the dotted blue line denotes the prediction reported in Ref.…”

“…The authors of Ref. [23,35], by means of a Langevin dynamics for heavy quarks coupled to hydrodynamic equations for the bulk medium, have highlighted for the first time that the v 1 of D mesons in noncentral collisions is several times larger than that of charged particles; their prediction (solid red line labelled as "Hydro+EM") is shown in Fig. 5a along with the result from A Multi-Phase Transport (AMPT) model [36] (dashed brown curve).…”

“…[17][18][19] and is adopted in further studies for describing the effect of the EMF on final observables; in particular Refs. [19,[22][23][24] have addressed the topic of the EMF influence on particle directed flow in HICs. However, the assumption of a constant conductivity during all the evolution of the heavy-ion collision may lead to overestimate the slowdown of the magnetic field decay, especially in the early stage when the medium is not yet produced.…”

“…In particular, we will discuss the experimental data for the directed flow of neutral D mesons in comparison to the theoretical expectations and to the measurements for light hadrons. Figure 5 shows the STAR data in 10-80% Au+Au collisions at RHIC energy of √ s NN = 200 GeV [65] along with predictions from theoretical models [22,23,35,36]. In panel (a) the rapidity dependence of the averaged directed flow Therefore, the STAR Collaboration observed an absolute value of the neutral D mesons dv 1 /dy that is about 25 times larger than that of the charged kaons with a 3.4σ significance [65] and is the largest among those of all the eleven particle species measured at top RHIC energy [28,30,32].…”

“…where the brakets • • • indicate the average over the particles in the collision, is related to a collective sidewards deflection of particles on the event plane with respect to the beam axis (v 1 ∼ p x / p T ) and the geometry of EMF and fluid velocities requires that the Lorentz force produces a net push of charges along the x axis with opposite directions for positively and negatively charges. Hence, the splitting of the directed flow between particles with the same mass and different electric charge gives direct access to the collective electromagnetic response of the medium [11,13,19,20,[22][23][24][25][26][27]. Furthermore, the measurements of the v 1 produced in different colliding systems as well as for different energies and centralities [28][29][30][31][32][33] give the opportunity to theoretical models to test their description of the initial state of highenergy collision, especially in view of the combined influence of EMF and vortical dynamics on the directed flow with a different sensitivity for bulk light particles and early-produced heavy quarks [11,13,19,20,[22][23][24][25][26][27][34][35][36].…”