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.

超相对论能量在核碰撞中产生的热而稠密的QCD物质的特征是非常强烈的电磁场，该电磁场在预平衡早期就达到了最大强度，并与碰撞产生的大角动量与等离子体中引起的强涡旋相互作用系统。可以观察到的这些现象的踪迹是在对称和非对称重离子碰撞以及质子诱导的反应中产生的轻质强子和重介子的有向流。尤其是，具有相同质量但电荷相反的粒子之间的定向流的分离，作为速度和横向动量的函数，可以访问所有碰撞阶段和不同碰撞系统中介质的电磁响应。预计在碰撞的平衡前阶段，电磁场的影响最大，因此，在早期生产的重夸克上会留下明显的烙印。这篇综述的目的是讨论对理解嵌入在大型和小型系统中的电磁场的产生和弛豫时间及其对轻和重粒子的电荷奇数定向流动的影响的理解的最新进展，重点是实验结果和不同的理论方法。由于可以对高能碰撞进行逼真的模拟，其中还包含所产生的电磁场和涡度，因此对有向流的研究可以提供对早期非平衡相以及随之而来的QGP形成和输运性质的独特见解。