Abstract
At the earliest times after a heavy-ion collision, the magnetic field created by the spectator nucleons will generate an extremely strong, albeit rapidly decreasing in time, magnetic field. The impact of this magnetic field may have detectable consequences, and is believed to drive anomalous transport effects like the Chiral Magnetic Effect (CME). We detail an exploratory study on the effects of a dynamical magnetic field on the hydrodynamic medium created in the collisions of two ultrarelativistic heavy-ions, using the framework of numerical ideal MagnetoHydroDynamics (MHD) with the ECHO-QGP code. In this study, we consider a magnetic field captured in a conducting medium, where the conductivity can receive contributions from the electromagnetic conductivity sigma and the chiral magnetic conductivity sigma _{chi }. We first study the elliptic flow of pions, which we show is relatively unchanged by the introduction of a magnetic field. However, by increasing the magnitude of the magnetic field, we find evidence for an enhancement of the elliptic flow in peripheral collisions. This effect is stronger at RHIC than the LHC, and it is evident already at intermediate collision centralities. Next, we explore the impact of the chiral magnetic conductivity on electric charges produced at the edges of the fireball. This initial sigma _chi can be understood as a long-wavelength effective description of chiral fermion production. We then demonstrate that this chiral charge, when transported by the MHD medium, produces a charge dipole perpendicular to the reaction plane which extends a few units in rapidity. Assuming charge conservation at the freeze-out surface, we show that the produced charge imbalance can have measurable effects on some experimental observables, like v_1 or langle sin phi rangle . This demonstrates the ability of a MHD fluid to transport the signature of the initial chiral magnetic fields to late times. We also comment on the limitations of the ideal MHD approximation and detail how further development of a dissipative-resistive model can provide a more realistic description of the QGP.
Highlights
The collision of two ultrarelativistic nuclei deposits enough energy such that the constituents of the nucleons become liberated, forming a strongly interacting plasma, the Quark Gluon Plasma (QGP); phenomenological studies suggest that this is the most perfect fluid created in nature [1]
We first study the elliptic flow of pions, which we show is relatively unchanged by the introduction of a magnetic field
We find that simple variations of σ0 are not sufficient to trigger significant modifications in v2 at mid-rapidity, compared to the case with no magnetic field, both at RHIC and LHC energies, not even for peripheral collisions
Summary
The collision of two ultrarelativistic nuclei deposits enough energy such that the constituents of the nucleons become liberated, forming a strongly interacting plasma, the Quark Gluon Plasma (QGP); phenomenological studies suggest that this is the most perfect fluid created in nature [1]. Despite the limits of our model, in this paper we will elucidate a number of novel implications of interacting magnetic fields and a perfect fluid on the observables of heavy-ion collisions Another exciting prospect is the ability to develop a greater understanding of anomalous transport phenomena in heavyion collisions, like the Chiral Magnetic Effect (CME) [2,34], the interest of the present study, and the Chiral Magnetic Wave [35]. While only the order of magnitude of the initial field is known, signatures of this strong field may aid in determining the relevant field strength To this end, in this paper, we study ideal (3+1)D MHD simulations using geometrical Glauber initial conditions [50] and we explore how varying basic parameters like the impact parameter, the conductivity of the medium in the pre-equilibrium phase, the freeze-out temperature and the magnitude of the initial magnetic field affects the elliptic flow of pions at mid-rapidity.
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