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.
Highlights
Phase of strongly-interacting matter: the Quark–Gluon Plasma (QGP)
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
In this review the main approaches adopted in theoretical calculations for describing the generation and time evolution of the electromagnetic fields (EMF) in ultrarelativistic nuclear collisions has been discussed, highlighting the significant differences in the fields produced in symmetric and asymmetric systems
Summary
In the last decades the surprising behaviour of the QGP related to its transport properties has been intensively studied in Heavy-Ion Collisions (HICs). Semi-analytic calculations of the magnetic and electric field evolution in a plasma with constant electric conductivity [17–19] indicate a strong slowdown of the field decay with respect to the evolution in the vacuum This description is not reliable in the very early stage, before any medium is produced, but suggests that, after dropping by some order of magnitude, as soon as the conducting matter is created the EMF freeze out in it and last as long as the QGP lifetime. The measurements of the v1 produced in different colliding systems as well as for different energies and centralities [28–33] 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 [11,13,19,20,22–27,34–36]. The presence of a magnetic field affects the QCD phase diagram, modifying the behaviour of the pseudocritical temperature and the position of the critical endpoint [46,47]
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