Exploring Electromagnetic Field effects using Charge Dependent Directed Flow at RHIC
Abstract
Ultra-strong electromagnetic fields generated in high-energy heavy-ion collisions could significantly influence the emission of charged particles in the final state. Charge-dependent directed flow (v1) makes a sensitive probe of the initial field strength and its evolution within the deconfined quark-gluon plasma (QGP). In this work, we present STAR measurements of Δv1, defined as the difference in v1 between π+ (K+, p, Λ) and π− (K−, \bar{p}, \bar{Λ}), as a function of rapidity, transverse momentum, and centrality. The data were obtained from Au+Au collisions at √sNN = 7.7, 9.2, 11.5, 14.6, 17.3, and 19.6 GeV during RHIC Beam Energy Scan Phase II. Our findings reveal a dominant Faraday+Coulomb effect in peripheral collisions and an enhanced electromagnetic influence at lower collision energies. Furthermore, the nonzero Δv1 observed for Λ and \bar{Λ}, together with the results for charged particles, strengthens the evidence for electromagnetic forces acting on constituent quarks within the QGP.
Figures
v1 and delta v1 as a function of rapidity
v1 and delta v1 as a function of pT
dv1/dy as a function of centrality
Beam energy dependence of deltav1 slope
Conclusion
We have presented STAR measurements of ∆dv1/dy for charged pions, charged kaons, (anti)protons, and (anti)Λ’s as a function of centrality in Au+Au collisions at six center-of-mass energies ranging from √sNN = 7.7 to 19.6 GeV. For charged particles, ∆dv1/dy becomes negative in peripheral collisions, with increasingly negative values at lower collision energies. This behavior is consistent with a dominant Faraday+Coulomb effect and a stronger electromagnetic influence at lower energies. Additionally, in peripheral collisions, Δv1 becomes more negative at higher pT, in agreement with theoretical expectations for the electromagnetic field. The observed Λ ∆dv1/dy follows a pattern consistent with a combination of proton and kaon ∆dv1/dy, supporting the hypothesis that electromagnetic forces act on constituent quarks prior to their coalescence into hadrons in the QGP. These findings provide critical insights into the strength and evolution of electromagnetic fields in heavy-ion collisions, the electrical conductivity of the QGP, and its response to strong electromagnetic fields.
- adash's blog
- Login or register to post comments