A Review on Intense Electromagnetic Fields in Heavy-Ion Collisions: Theoretical Predictions and Experimental Results

TL;DR

This article addresses the problem of characterizing ultra-strong electromagnetic fields generated in relativistic heavy-ion collisions and their coupling to the quark-gluon plasma. It synthesizes theoretical predictions for field generation via spectator charges, event-by-event fluctuations, and the field’s time evolution under a conducting QGP, alongside experimental evidence from ultra-peripheral collisions, charge-dependent directed flow, and global polarization. The review highlights large model uncertainties in conductivity and pre-equilibrium dynamics, and discusses how observed signatures constrain the field’s lifetime and strength. It also outlines challenges and opportunities, including new observables and upcoming high-statistics data to map field dynamics. Overall, the work provides a comprehensive reference for researchers studying EM-field effects in QCD matter.

Abstract

In heavy-ion collisions at relativistic energies, the incident nuclei travel at nearly the speed of light. These collisions deposit kinetic energy into the overlap region and create a high-temperature environment where hadrons ``melt'' into deconfined quarks and gluons. The spectator nucleons, which do not undergo scatterings, generate an ultra-intense electromagnetic field -- on the order of $10^{18}$ Gauss at Relativistic Heavy-Ion Collider, and $10^{19}$ Gauss at the Large Hadron Collider. These powerful electromagnetic fields have a significant impact on the produced particles, not only complicating the study of particle interactions but also inducing novel physical phenomena. To explore the nature of these fields and their interactions with deconfined quarks, we provide a detailed overview, encompassing theoretical estimations of their generation and evolution, as well as experimental efforts to detect them. We also provide physical interpretations of the discovered results and discuss potential directions for future investigations.

Figures (18)

• Figure 1: Sketch of a heavy-ion collision in the lab frame (figure is from Ref. STAR:2023jdd). The impact parameter and the beam direction are along the $x$ and $z$ axes, respectively. Spectator nuclear fragments generate strong magnetic fields along $-y$.
• Figure 2: Magnetic field produced mainly by spectator protons in semi-central Au+Au collisions at $\sqrt{s_{NN}}=$200 GeV, without medium responses (figures are from Refs. Voronyuk:2011jdSkokov:2009qpKharzeev:2007jp).
• Figure 3: The peak value of the magnetic field strength as a function of (a) collision energy and (b) impact parameter (figure is from Ref. Siddique:2021smf).
• Figure 4: Electromagnetic fields in Au+Au collisions at $\sqrt{s_{NN}}=$ 200 GeV as functions of impact parameter $b$. The magnetic- and electric-field fluctuations are indicated by the magnitude averages along different directions. (a) Calculations using initial condition from the Heavy Ion Jet INteraction Generator (HIJING) model (figure is from Ref. Deng:2012pc). (b) The initial nucleon distribution is from the Woods-Saxon distribution with standard parameters, assuming both nuclei are infinitely thin (figure is from Ref. Bzdak:2011yy).
• Figure 5: Correlations between magnetic field and second-harmonic participant plane in Au+Au, Ru+Ru, and Zr+Zr collisions at $\sqrt{s_{NN}}=$ 200 GeV (figure is from Ref. Zhao:2019crjBloczynski:2012en). The more negative $\langle \cos n(\Psi_B -\Psi_2)\rangle$ indicates a stronger correlation between the magnetic field and the participant plane $\Psi_2$.