Unlocking the nuclear secrets of the universe's strongest magnetic fields

Data from heavy ion collisions provide new insight into the quark-gluon's electromagnetic properties Plasma.

New analysis by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), a particle collider at the US Department of Energy's (DOE) Brookhaven National Laboratory, provides the first direct evidence of the universe's most powerful signature. Magnetic fields in the case of a „confined” nucleus. The evidence comes from measuring the way differently charged particles split apart when emitted from collisions of nuclei at this DOE Office of Science user facility.

As described in the journal Physical examination X, the powerful magnetic fields generated in off-center collisions induce currents in quarks and gluons, liberated by particle smashups from protons and neutrons, or confined. The findings offer scientists a new way to study the electrical conductivity of this „quark-gluon plasma” (QGP).

„This is the first measurement of how the magnetic field interacts with the quark-gluon plasma (QGB),” said Tiu Shen, a STAR physicist at China's Fudan University and leader of the new analysis. Indeed, measuring the impact of those interactions provides direct evidence that these powerful magnetic fields exist.

Using the STAR detector, scientists can track the paths of particles emitted from heavy ion collisions in the Relativistic Heavy Ion Collider (RHIC). This composite image shows high-energy house-size detectors and sample particle tracks from gold-gold collisions. Credit: Brookhaven National Laboratory

A neutron is more powerful than a star

Scientists have long believed that collisions outside the core of heavy nuclei such as gold, also known as heavy ions, create powerful magnetic fields. That's because the few positively charged protons—and neutral neutrons—that don't collide form nuclei, and the ions spin around each other at near the speed of light.

„Those fast-moving positive charges should create a very strong magnetic field, which is predicted to be 10¹⁸ said star physicist Gang Wang of the University of California, Los Angeles. By comparison, neutron stars, the densest matter in the universe, have about 10 fields, he noted.¹⁴ gauss, refrigerator magnets produce a field of about 100 gauss, and our planet's shielding magnetic field is only 0.5 gauss. „It's probably the strongest magnetic field in our universe.”

But because things happen so quickly in high-ion collisions, the field doesn't last long. It scatters less than 10^-23 Seconds—billionths of a second—billionths of a second—make it hard to notice.

So instead of trying to measure the field directly, the STAR scientists looked for evidence of its impact on the particles ejected from the collisions.

„Specifically, we were looking at the collective motion of charged particles,” Wang said.

Top-down view of an off-center heavy-ion collision. The rapid decay of the ultra-strong magnetic field (B) induces current through Faraday induction, which affects the trajectory of the charged particles. The amount of deflection is directly related to the conductivity of the quark-gluon plasma (QGP), giving scientists a way to measure this important property. Credit: Thieu Shen/Fundan University

Deviation detection

It is well known that magnetic fields can affect the motion of charged particles and induce electromagnetic fields in conductive forms of matter such as metals. That's what's happening here, but on a much smaller scale.

„We wanted to see if the charged particles produced in off-center heavy ion collisions are deflected in a way that can only be explained by the presence of an electromagnetic field in the small points of the QGP created in these collisions,” said Eihong Tang. , Brookhaven Laboratory physicist and member of the STAR collaboration.

The team used STAR's sophisticated detection systems to monitor the collective motion of different pairs of charged particles while ruling out the influence of competing non-electromagnetic effects. They were most interested in ruling out deflections caused by charged quarks carried as part of the colliding nuclei. Fortunately, those „transported quarks” produce a deflection pattern opposite to that induced by a current induced by a magnetic field, known as Faraday induction.

A clear signal

„Finally, we see a charge-dependent deflection mode induced only by the electromagnetic field in the QGP—a clear indication of Faraday induction,” Tang said.

The scientists observed this strong signal not only in off-center collisions of two gold nuclei at high energies—gold-gold at 200 billion electron volts, or GeV—but also in off-center collisions of smaller nuclei—ruthenium-ruthenium and zirconium. Zirconium, both at 200 GeV.

„This effect is universal. It occurs not only in a large system, but also in a small system,” Shen said.

Scientists found an even stronger signal when they analyzed data from gold-gold collisions at a relatively low energy – 27 GeV. This finding provides additional evidence that the particle-changing electromagnetic field is induced by powerful magnetic fields created by off-center collisions.

This is because Faraday induction occurs when the magnetic field dissipates. In low-energy collisions, it happens much more slowly.

„This effect is stronger at lower energies because the lifetime of a magnetic field is longer at lower energies; because the speed of nuclear fragments is lower, the magnetic field and its effects last longer,” Wang said.

Impacts

Now that scientists have evidence that magnetic fields induce an electromagnetic field in the QGP, they can use the induction to study the conductivity of the QGP.

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

Understanding the fundamental electromagnetic properties of QGP can provide insights into important questions in physics. For one, magnetic fields that induce electromagnetic effects may contribute to interesting separation of particles according to their „handedness” or chirality.

„This study provides strong evidence of a magnetic field, which is one of the prerequisites for this.”Chiral magnetic effect,'” Shen said.

The QGP's magnetic field and electromagnetic properties play a role in determining the conditions under which free, confined quarks and gluons can combine to form composite particles called hadrons—like the protons and neutrons that make up ordinary nuclei.

„We want to map the nuclear 'phase diagram,' which shows at what temperature quarks and gluons can be considered free and at what temperature they 'freeze' into hadrons. Those properties and the fundamental interactions of quarks and gluons mediated by the strong force are modified under an intense electromagnetic field,” Wang said. said. With this new study of QGP's electromagnetic properties, he added, „these fundamental properties can be explored in another dimension to provide more information about the strong interaction.”

For now, the scientists pointed out, theorists will look to these results to refine explanations.

Reference: “Monitoring the Electromagnetic Field Effect in Charge-Ion Collisions by Charge-Dependent Directed Flow”, by 23 February 2024. Physical examination X.
DOI: 10.1103/PhysRevX.14.011028

This research was funded by the DOE office, the US National Science Foundation, and international organizations and agencies listed in the Science Paper. The STAR team utilized computing resources at the Science Data and Computing Center at Brookhaven Laboratory, the National Energy Research Science Computing Center (NERSC) at DOE's Lawrence Berkeley National Laboratory, and the Open Science Grid Consortium.