The search for magnetic-induced charged currents in Pb--Pb collisions with ALICE

TL;DR

This paper investigates the potential electromagnetic effects of strong magnetic fields in heavy-ion collisions by measuring charge-dependent directed flow in Pb--Pb collisions at 5.02 TeV, aiming to distinguish pure electromagnetic signals from QCD anomalies.

Contribution

It provides the first experimental test of a predicted electromagnetic current effect induced by magnetic fields in the Quark-Gluon Plasma, separate from the Chiral Magnetic Effect.

Findings

No significant charge-dependent directed flow asymmetry observed.

Results help calibrate magnetic field effects in heavy-ion collision experiments.

Data constrains models of electromagnetic phenomena in QGP.

Abstract

In non-central heavy-ion collisions unprecedented strong magnetic fields, of the order of 10$^{18}$ Gauss, are expected to be produced. The interplay of such fields with QCD anomalies in the Quark--Gluon Plasma (QGP) has been predicted to lead to a number of interesting phenomena, such as the Chiral Magnetic Effect (CME). While several experimental observations are partially consistent with predictions of a CME signal, it is hard to distinguish them unambiguously from a combination of more mundane phenomena present in the anisotropic expansion of the QGP. This makes it imperative to establish that the early-time magnetic field has observable consequences not related to the anomalous QCD effects on final-state charged particles and to calibrate its strength. We test a recent prediction of a pure electromagnetic effect, which may arise in heavy-ion collisions. The varying magnetic field would induce a current within the QGP, which is expected to leave a very peculiar imprint on final-state particles: a contribution to directed flow which is asymmetric both in charge and pseudorapidity. We report the measurement of the charge dependence of directed flow with respect to the spectator plane for unidentified charged particles in Pb--Pb collisions at $\sqrt{s_\text{NN}} = 5.02$ TeV.