Event-by-event generation of electromagnetic fields in heavy-ion collisions

Abstract: We compute the electromagnetic fields generated in heavy-ion collisions by using the HIJING model. Although after averaging over many events only the magnetic field perpendicular to the reaction plane is sizable, we find very strong magnetic and electric fields both parallel and perpendicular to the reaction plane on the event-by-event basis. We study the time evolution and the spatial distribution of these fields. Especially, the electromagnetic response of the quark-gluon plasma can give non-trivial evolution of the electromagnetic fields. The implications of the strong electromagnetic fields on the hadronic observables are also discussed.

• The paper demonstrates that individual collision events produce extremely strong electric and magnetic fields, reaching up to 10^18 G.
• It details the temporal and spatial evolution of the fields, from pre-collision increases to rapid post-overlap decay affected by transverse momenta.
• The study reveals how quark-gluon plasma conductivity alters field longevity, prompting refined magnetohydrodynamic models for heavy-ion collisions.

Event-by-event Generation of Electromagnetic Fields in Heavy-ion Collisions

This paper presents a detailed study of the electromagnetic fields generated during relativistic heavy-ion collisions. Using the HIJING (Heavy Ion Jet INteraction Generator) model, the authors compute the electromagnetic (EM) fields on an event-by-event basis, focusing on the temporal and spatial characteristics of these fields. Previous studies have primarily addressed the fields averaged over many events, which led to limited insights into the notable fluctuations that can arise from individual collisions. This investigation provides valuable insights into the behavior of EM fields in high-energy collisions, such as those performed at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).

Overview of Key Findings

• Strong EM Fields on an Event-by-event Basis: Contrary to event-averaged expectations, the study finds that individual heavy-ion collision events can generate very strong electric and magnetic fields, both parallel and perpendicular to the reaction plane. The magnetic fields in particular can reach up to $10^{18}$ G, significantly higher than what is typically achieved in laboratory settings.
• Temporal and Spatial Evolution: The paper explores the time evolution and spatial distribution of these EM fields. The fields exhibit an increase before the collision as the nuclei approach, followed by a rapid decay post-overlap as the spectators recede. The remnants of the collision, which possess non-trivial transverse momenta, continue to influence the fields over longer durations.
• Impact of Quark-Gluon Plasma (QGP): The research also explores how the QGP, potentially formed in these collisions, can influence the evolution of the electromagnetic fields. Given its electrical conductivity, the QGP can have a non-trivial response, affecting the longevity and strength of these fields.
• Fluctuations and Observables: The authors illustrate that the fluctuations in the particles' positions at the time of collision lead to significant variances in the strength and direction of both electric and magnetic fields. Such fluctuations have essential implications for understanding various phenomena observed in heavy-ion collisions, such as the chiral magnetic effect in quantum chromodynamics (QCD).

Numerical and Theoretical Findings

The results emphasize the profound magnitude and orientation fluctuations of EM fields, underpinned by systematic numerical simulations via the HIJING model. Notably, the study reveals that although $B_y$ (the field component perpendicular to the reaction plane) is dominant on average, components like $B_x$ can be equally significant for individual events due to geometric fluctuations. The fields are also shown to scale linearly with the collision energy $\sqrt{s}$, providing a predictive framework for their behavior at different collisional energies.

Implications and Future Directions

The findings underscore the necessity of considering event-by-event fluctuations when examining observables related to EM fields in heavy-ion collisions. These fluctuations could substantially affect signals sensitive to the dynamic electromagnetic background, such as charge separation or elliptic flow differences between charged particles. Additionally, the paper boosts our comprehension of the QGP's effects on the fields' evolution, positing a foundation for more detailed magnetohydrodynamic treatments in the future.

Given the complex interactions between EM fields and the collision dynamics, future research could extend these findings by studying the feedback mechanisms between the fields and the medium more comprehensively. Advanced computational models incorporating a dynamically evolving QGP would yield further insights into the rich phenomenology of strong-interaction fields in heavy-ion physics.