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Strong magnetic fields leave marks on nuclear matter

Scientists have made the first direct observation of the influence of the universe’s most potent magnetic fields on deconfined nuclear matter.

This discovery, unveiled in a new study by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, emerged from examining how particles of opposite charges diverge after being released from collisions of atomic nuclei.

This finding offers a novel method to probe the electrical conductivity of quark-gluon plasma (QGP), shedding light on the fundamental components of atomic nuclei.

First direct evidence of cosmic magnetic fields

Diyu Shen, a leading physicist from Fudan University and a key member of the STAR collaboration, emphasized the significance of this research, saying, “This is the first measurement of how the magnetic field interacts with the quark-gluon plasma (QGP).”

The study focuses on the effects of powerful magnetic fields created during off-center collisions of heavy atomic nuclei, such as gold.

These fields are believed to be among the strongest in the universe, significantly surpassing those found around neutron stars or generated by human-made objects.

“Those fast-moving positive charges should generate a very strong magnetic field, predicted to be 1018 gauss,” said Gang Wang, a STAR physicist from the University of California, Los Angeles. “This is probably the strongest magnetic field in our universe.”

Detecting magnetic fields in nuclear matter

Due to the fleeting nature of these magnetic fields, lasting less than a ten-millionth of a billionth of a billionth of a second, the researchers did not measure the fields directly.

Instead, they detected their impact on the particles ejected from the collisions. By analyzing the collective movement of charged particles, they sought to identify patterns indicative of electromagnetic field influence.

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

The research team meticulously tracked the movement of various pairs of charged particles, differentiating the effects of the magnetic fields from other potential influences.

This careful analysis led to the identification of charge-dependent deflection patterns unique to electromagnetic fields within the QGP, confirming the phenomenon of Faraday induction.

Observing effects in various atomic collisions

This effect was not limited to collisions involving large nuclei. It was also observed in smaller systems, indicating a universal principle.

“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,” said Aihong Tang, a Brookhaven Lab physicist and member of the STAR collaboration.

The signal was even stronger in lower-energy collisions, providing additional evidence of the magnetic fields’ role in inducing particle-deflecting electromagnetic fields.

“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,” said Wang.

The implications of this discovery extend far beyond the identification of these magnetic fields. By using the observed effects to probe the QGP’s conductivity, the scientists hope to uncover more about the fundamental properties of quarks and gluons.

Probing the QGP’s conductivity and beyond

“This is a fundamental and important property,” said Shen. “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.”

Furthermore, this research could offer insights into the chiral magnetic effect and the conditions under which quarks and gluons form hadrons, contributing to our understanding of the nuclear phase diagram and the strong force’s interactions under extreme electromagnetic fields.

Wang envisions this method as a new avenue to explore these fundamental properties, adding another dimension to our comprehension of the strong interaction.

“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,” Wang explained.

“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,” said Wang. We can investigate these fundamental properties in another dimension to provide more information about the strong interaction,” Wang concluded.

Fundamental physics of magnetic fields and nuclear matter

In summary, the STAR collaboration at the Relativistic Heavy Ion Collider has achieved a remarkable breakthrough, providing the first direct evidence of the universe’s most intense magnetic fields influencing deconfined nuclear matter.

By meticulously analyzing the deflection of charged particles emerging from atomic nuclei collisions, the researchers have opened new avenues for studying the electrical conductivity of quark-gluon plasma.

This pioneering work deepens our understanding of the fundamental forces that shape our universe and sets the stage for future investigations into the properties of quarks, gluons, and the complex interactions governed by the strong force.

With this discovery, the team at Brookhaven National Laboratory has taken a significant step forward in unlocking the mysteries of the cosmos, offering insights that could reshape our comprehension of the fundamental building blocks of matter.

The full study was published in the journal Physical Review X.


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