Charge-dependent directed flow in asymmetric nuclear collisions

Abstract

The directed flow of identified hadrons is studied within the parton-hadron-string-dynamics (PHSD) approach for the asymmetric system Cu+Au in non-central collisions atGeV. It is emphasized that due to the difference in the number of protons of the colliding nuclei an electric field emerges which is directed from the heavy to the light nucleus. This strong electric field is only present for about 0.25 fm/c at √ sNN = 200 GeV and leads to a splitting of the directed flow v1 for particles with the same mass but opposite electric charges in case of an early presence of charged quarks and antiquarks. The microscopic calculations of the directed flow for π ± , K ± , p andp are carried out in the PHSD by taking into account the electromagnetic field induced by the spectators as well as its influence on the hadronic and partonic quasiparticle trajectories. It is shown that the splitting of the directed flow as a function of pseudorapidity η and in particular as a function of the transverse momentum pt provides a direct access to the electromagnetic response of the very early (nonequilibrium) phase of relativistic heavy-ion collisions and allows to shed light on the presence (and number) of electric charges in this phase.PACS numbers: 25.75.-q, 25.75.Ag

Electromagnetic fields and anomalous transports in heavy-ion collisions—a pedagogical review

“…The EM fields in these collision systems were also studied [9,[12][13][14]100]. We here give a brief summary of these studies.…”

“…Very recently, there were interesting proposals for the novel effects of this in-plane E field in Cu + Au collisions, for example: the E field can lead to a directed flow v 1 splitting between positively and negatively charged hadrons [12,14], the presence of the in-plane E field may strongly suppress or even reverse the sign of the charge-dependent correlation γ αβ (see Eq. (78) for its definition) [13], and Cu + Au may serve to test the chiral electric separation effect [101]; more relevant discussions are given in Sec.…”

Charge-dependent anisotropic flow in Cu + Au collisions

“…The ∆v 1 is smaller than the model prediction, which indicates that the electric charges existing within the life time of the EF (∼0.25 fm/c) are much smaller than that of all quarks created in the collisions. Figure 2 shows v 1 and ∆v 1 as a function of η in 10-40% centrality 2 , where p T is integrated over 1<p T <2 GeV and the sign of Ψ Au 1 is defined to be negative (opposite to Fig. 1).…”

“…more positively charged particles would move to the direction of the EF and negatively charged particles move to the opposite side. The parton-hadron-string-dynamics (PHSD) model calculations with and without the effect of the EF [2] are compared to the data, where the ∆v 1 for the calculation including the effect of the EF is scaled by 0.1. The model assumes that all electric charges are affected by the EF, resulting in a large separation of v 1 between positive and negative charges as shown in the upper left panel of Fig.…”

“…The lifetime of the electric field might be short, of the order of a fraction of a fm/c. The quarks and antiquarks that have been already produced at this time would experience the Coulomb force, which results in a charge dependence of particle directed flow [1,2]. Thus, the measurement of the charge-dependent directed flow in Cu+Au collisions provides an opportunity to test different quark (charge) production scenarios, e.g.…”