Abstract
The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Noncentral collisions can produce strong magnetic fields on the order of , which offers a probe into the electrical conductivity of the QGP. In particular, quarks and antiquarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as . Here, we present the charge-dependent measurements of near midrapidities for , , and in and isobar ( and ) collisions at , and in collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the signal on collision system, particle species, and collision centrality can be qualitatively and semiquantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the and quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations.
- Received 10 April 2023
- Revised 14 December 2023
- Accepted 11 January 2024
DOI:https://doi.org/10.1103/PhysRevX.14.011028
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Focus
FocusColossal Magnetic Field Detected in Nuclear Matter
Published 23 February 2024
Collisions of heavy ions briefly produced a magnetic field times stronger than Earth’s, and it left observable effects.
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Popular Summary
In ultrarelativistic nuclear collisions, heavy ions such as gold and lead are accelerated close to the speed of light and pass through each other via collisions to create a deconfined quark-gluon plasma. When the collision is noncentral, an extremely intense electromagnetic field can be generated in the interaction region. This electromagnetic field has the highest strength ever achieved on Earth, but an extremely short lifetime. That hinders its direct observations by experiments. We examine the collective motion of charged hadrons emitted from the plasma to uncover the characteristic pattern imprinted by that electromagnetic field.
In this work, we analyze data on ion collisions collected at Brookhaven’s Relativistic Heavy Ion Collider in 2014 and 2016 as part of the STAR experiment. We measure, with unprecedented precision, the charge-dependent sideward motion of mesons, mesons, and protons emitted from those collisions. These motions provide, for the first time, a clear signature in glancing collisions of both Faraday induction—an electric field induced by the fast decay of the magnetic field in the quark-gluon plasma—and the Coulomb effect—acceleration due to the electric fields of other protons in the vicinity. This combination confirms the interaction between the strong electromagnetic field and the quark-gluon plasma.
These results pave the way for investigation of the properties of the plasma and other novel phenomena that are fundamental to our understanding of the strong nuclear interaction.
