Measuring the Conductivity of the Universe's Hottest Matter

The STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), located at the US Department of Energy’s (DOE) Brookhaven National Laboratory, has conducted a new analysis. This analysis offers the initial direct evidence of the influence of potentially the universe’s most formidable magnetic fields on "deconfined" nuclear matter.

This evidence is derived from the observation of the distinct separation patterns of differently charged particles following collisions of atomic nuclei at this DOE Office of Science user facility.

The data show that strong magnetic fields produced in off-center collisions cause an electric current to be induced in the quarks and gluons that are released, or deconfined, from protons and neutrons by the particle smashups, as reported in the journal Physical Review X. To understand more about these essential components of atomic nuclei, scientists can now investigate the electrical conductivity of this “quark-gluon plasma” (QGP) thanks to the findings.

This is the first measurement of how the magnetic field interacts with the quark-gluon plasma (QGP).

Diyu Shen, STAR Physicist and Study Lead, Fudan University

Measuring the impact of that interaction provides direct evidence that these powerful magnetic fields exist.

More Powerful Than a Neutron Star

For a considerable time, scientists have held the belief that strong magnetic fields could be produced when heavy atomic nuclei, such as gold or heavy ions, collide off-center. This is because when the ions sideswipe one another at nearly the speed of light, some of the positively charged protons and neutral neutrons that make up the nuclei would be set aswirl.

Those fast-moving positive charges should generate a very strong magnetic field, predicted to be 10¹⁸ gauss.

Gang Wang, STAR Physicist, University of California

For comparison, Wang noted that neutron stars, the densest objects in the universe, have fields of about 10¹⁴ gauss, while refrigerator magnets produce a field of about 100 gauss, and Earth’s protective magnetic field measures a mere 0.5 gauss. “This is probably the strongest magnetic field in our universe.”

However, the field is transient in heavy ion collisions due to the rapidity of events. It is difficult to observe because it dissipates in less than 10-23 seconds, or ten-millionths of a billionth of a second.

Hence, rather than directly measuring the field, the STAR scientists searched for signs of its influence on the particles emerging from the collisions.

Specifically, we were looking at the collective motion of charged particles.

Gang Wang, STAR Physicist, University of California

Detecting Deflection

It is commonly known that magnetic fields can influence charged particle motion and that conductive materials like metals can even produce electromagnetic fields. On a far smaller scale, that is precisely what occurs here.

“We wanted to see if the charged particles generated in off-center heavy ion collisions were being deflected in a way that could only be explained by the existence of an electromagnetic field in the tiny specks of QGP created in these collisions,” noted Aihong Tang, a Brookhaven Lab physicist, and member of the STAR collaboration.

The researchers eliminated the impact of competing non-electromagnetic effects by tracking the collective motion of several pairs of charged particles using STAR’s advanced detector systems. The most important thing they wanted to do was rule out deflections brought about by charged quarks carried into the colliding nuclei. Thankfully, the deflection pattern caused by the “transported quarks” is the opposite of the Faraday induction pattern caused by the electric current induced by the magnetic field.

A Clear Signal

Tang says, “In the end, we see a pattern of charge-dependent deflection that can only be triggered by an electromagnetic field in the QGP—a clear sign of Faraday induction.”

This robust signal was observed by the scientists in off-center collisions of smaller nuclei (ruthenium-ruthenium and zirconium-zirconium, both at 200 GeV) as well as off-center collisions of two high-energy gold nuclei (gold-gold at 200 billion electron volts, or GeV).

Shen says, “This effect is universal. It happens not just in a big system but also in a smaller system.”

When the researchers examined data from gold-gold collisions at a comparatively low energy of 27 GeV, they observed an even stronger signal. This result adds to the body of evidence suggesting that the strong magnetic fields produced by off-center collisions were responsible for inducing the particle-deflecting electromagnetic field.

This is because when the magnetic field weakens, Faraday induction takes place. That occurs more slowly in collisions with lower energy.

Wang says, “This effect is stronger at lower energy because the lifetime of magnetic field is longer at lower energy; the speed of the nuclear fragments is lower, so the magnetic field and its effects last longer.”

Implications

Scientists can now investigate the conductivity of the QGP by using the evidence that magnetic fields cause an electromagnetic field to be induced within it.

Shen says, “This is a fundamental and important property. We can infer the value of the conductivity from our measurement of the collective motion. The extent to which the particles are deflected relates directly to the strength of the electromagnetic field and the conductivity in the QGP—and no one has measured the conductivity of QGP before.”

Comprehending the fundamental electromagnetic characteristics of the Quark-Gluon Plasma (QGP) could provide valuable insights into significant physics inquiries. Notably, the magnetic fields responsible for inducing electromagnetic phenomena might play a role in intriguingly segregating particles based on their "handedness," or chirality.

Shen says, “This study gives strong evidence of the magnetic field, which is one of the preconditions for this ‘chiral magnetic effect.”

The magnetic field and electromagnetic properties of the QGP also play a role in determining the conditions under which free, deconfined quarks and gluons coalesce to form composite particles called hadrons—such as the protons and neutrons that make up ordinary nuclei.

Wang says, “We want to map out the nuclear ‘phase diagram,’ which shows at which temperature the quarks and gluons can be considered free and at which temperature they will ‘freeze out’ to become hadrons. Those properties and the fundamental interactions of quarks and gluons, which are mediated by the strong force, will be modified under an extreme electromagnetic field.”

With this new probe of the QGP’s electromagnetic properties, Wang adds, “We can investigate these fundamental properties in another dimension to provide more information about the strong interaction.”

The scientists noted that theorists will be examining these findings to help improve the interpretations for the time being.

The Office of Science of the US Department of Energy supports Brookhaven National Laboratory. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of this time.

Journal Reference:

Abdulhamid, M. I., et al. (2024) Observation of the Electromagnetic Field Effect via Charge-Dependent Directed Flow in Heavy-Ion Collisions at the Relativistic Heavy Ion Collider. Physical Review X. doi.org/10.1103/PhysRevX.14.011028