Magnetic effects in heavy-ion collisions at intermediate energies

TL;DR

This study investigates electromagnetic fields in heavy-ion collisions at intermediate energies, revealing that magnetic fields significantly influence pion ratios and should be considered when probing nuclear symmetry energy.

Contribution

It demonstrates the impact of magnetic fields on pion ratios in heavy-ion collisions and highlights their importance in studying nuclear symmetry energy at high densities.

Findings

Magnetic fields reach up to 7×10^{16} G but have minimal effect on nucleon observables.

Magnetic fields strongly affect the differential π^-/π^+ ratio at forward/backward rapidities.

The differential pion ratio is sensitive to the density dependence of nuclear symmetry energy.

Abstract

The time-evolution and space-distribution of internal electromagnetic fields in heavy-ion reactions at beam energies between 200 and 2000 MeV/nucleon are studied within an Isospin-dependent Boltzmann-Uhling-Uhlenbeck transport model IBUU11. While the magnetic field can reach about $7\times 10^{16}$ G which is significantly higher than the estimated surface magnetic field ($\sim 10^{15}$ G) of magnetars, it has almost no effect on nucleon observables as the Lorentz force is normally much weaker than the nuclear force. Very interestingly, however, the magnetic field generated by the projectile-like (target-like) spectator has a strong focusing/diverging effect on positive/negative pions at forward (backward) rapidities. Consequently, the differential $\pi^-/\pi^+$ ratio as a function of rapidity is significantly altered by the magnetic field while the total multiplicities of both positive and negative pions remain about the same. At beam energies above about 1 GeV/nucleon, while the integrated ratio of total $\pi^-$ to $\pi^+$ multiplicities is not, the differential $\pi^-/\pi^+$ ratio is sensitive to the density dependence of nuclear symmetry energy $E_{\rm{sym}}(\rho)$. Our findings suggest that magnetic effects should be carefully considered in future studies of using the differential $\pi^-/\pi^+$ ratio as a probe of the $E_{\rm{sym}}(\rho)$ at supra-saturation densities.