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

Abstract

The hot and dense QCD matter produced in nuclear collisions at ultrarelativistic energy is characterized by very intense electromagnetic fields which attain their maximal strength in the early pre-equilibrium stage and interplay with the strong vorticity induced in the plasma by the large angular momentum of the colliding system. A promising observable keeping trace of these phenomena is the directed flow of light hadrons and heavy mesons produced in symmetric and asymmetric heavy-ion collisions as well as in proton-induced reactions. In particular, the splitting of the directed flow between particles with the same mass but opposite electric charge as a function of rapidity and transverse momentum gives access to the electromagnetic response of medium in all collision stages and in the different colliding systems. The highest influence of the electromagnetic fields is envisaged in the pre-equilibrium stage of the collision and therefore a significant imprint is left on the early-produced heavy quarks. The aim of this review is to discuss the current developments towards the understanding of the generation and relaxation time of the electromagnetic fields embedded in both large and small systems and their impact on the charge-odd directed flow of light and heavy particles, highlighting the experimental results and the different theoretical approaches. Since it is possible to perform realistic simulations of high-energy collisions that incorporate also the generated electromagnetic fields and vorticity, the study of the directed flow can provide unique insight into the early nonequilibrium phase and the ensuing QGP formation and transport properties.

Directed flow of D mesons at RHIC and LHC: non-perturbative dynamics, longitudinal bulk matter asymmetry and electromagnetic fields

“…Indeed, the fraction of the orbital angular momentum of the colliding system transferred to the hot plasma manifests itself with swirls of the order of 2 − 3 fm −1 whose observable effects could manifest in the chiral vortical effect [14][15][16][17], the spin polarization of baryons and vector mesons [18][19][20][21][22][23][24] and the "wiggle" slope in the directed flow of hadrons [25][26][27][28]. Furthermore, electromagnetic fields of the order of few to tens of m 2 π are produced in the overlap area mainly due to the motion of spectator charges and can be probed by observations of the chiral magnetic effect and related quantum phenomena [7,[29][30][31], the splitting in the spin polarization [32][33][34] and the splitting of the directed flow (a dipole asymmetry) [35][36][37][38][39][40][41][42][43][44]; more recently also a possible impact on v 2 , v 3 and the average transverse momentum < p T > has been pointed out [39]. In the last 15 years there has been a large effort to study the interaction of heavy quarks (HQs) with the bulk medium [45][46][47][48][49][50][51][52]…”

Manipulating strong electromagnetic fields with the average transverse momentum of relativistic nuclear collisions

“…This new method of inducing phenomenological manifestations of the strong magnetic field brings the following remarkable advantages. 4 We do not understand why our result becomes so significantly negative around 20% centrality for the S/A e predictor. This requires further investigation with realistic hydrodynamic simulations.…”

“…The observable that seems intrinsically connected with the manifestation of the strong B field in heavy-ion collisions is the dipolar flow of charged hadrons, v 1 , as recently discussed in Ref. [4]. The CME is indeed a charge-dependent v 1 , driven by an electric current flowing along the B field direction in presence of local parity violation [5].…”

Directed flow of D mesons at RHIC and LHC energy within a transport approach: non-perturbative dynamics, vorticity and electromagnetic fields

“…In these first theoretical investigations a Langevin approach coupled to the Maxwell equations have been used. Recent experimental data from STAR collaboration [3] at top RHIC energies and from ALICE collaboration [4] at LHC energies have shown a directed flow larger than the one of light charged hadrons [5]. Recently, in Ref.s [6,7], important features of the directed flow splitting ∆v 1 induced by e.m. fields have been found suggesting that measurement of ∆v 1 of D mesons and leptons form Z 0 decay and their correlations could be powerful tool to probe the initial e.m. fields in uRHICs.…”