When two beams of gold ions collide at the Relativistic Heavy Ion Collider, they create the quark-gluon plasma. Shortly after the collision, the particles stream out to several detectors. This is the end view from STAR, one of four detectors.
Image Credit: Brookhaven National Laboratory
(CC BY-NC-ND 3.0)
Scientific Frontline: Extended "At a Glance" Summary: Heavy Quark Transport in Quark-Gluon Plasma
The Core Concept: Quark-gluon plasma is a trillion-degree fluid of subatomic particles that existed in the first fraction of a second after the Big Bang. To understand its microscopic structure, physicists track the momentum changes of heavy quarks as they travel through the plasma at near-light speeds.
Key Distinction/Mechanism: Traditional Brownian motion models assume a symmetric distribution of microscopic forces, meaning extreme kicks to a particle are equally likely in all directions. In relativistic conditions, this breaks down; heavy quarks experience an uneven distribution of forces, making extreme, slowing kicks significantly more likely than accelerating ones, which indicates the plasma behaves more like a strongly interacting liquid than a gas.
Origin/History: Albert Einstein established the foundational theory of Brownian motion in 1905 to describe how particles move through fluid. In November 2026, researchers from the Massachusetts Institute of Technology, the University of California, Santa Barbara, and the Kavli Institute for Theoretical Physics published a new theory in Physical Review Letters extending this concept to relativistic velocities, extreme temperatures, and subatomic scales.
Major Frameworks/Components:
- Brownian motion: The baseline model of microscopic drag and random particle kicks, which was originally used to deduce fluid properties.
- Strong nuclear force: The fundamental interaction that binds quarks and gluons, used to calculate the momentum changes of heavy quarks.
- Relativistic kinematics: The physics of particles moving at velocities approaching the speed of light, where standard fluid dynamics calculations break down.
- Non-Gaussian probability distribution: The mathematical curve describing the uneven, tilted frequency of large momentum changes experienced by relativistic particles.
Branches of Science: Quantum Chromodynamics, High-Energy Particle Physics, Theoretical Fluid Dynamics, and Nuclear Physics
Future Application: These theoretical models will be compared with experimental data from large particle accelerators, such as the Relativistic Heavy Ion Collider, to deduce the exact microscopic structure of the primordial fluid.
Why It Matters: This framework provides a new, concrete method for using heavy quarks as probes to understand the extreme conditions, physical dynamics, and fundamental interactions that characterized the universe mere microseconds after the Big Bang.
In the first fraction of a second following the Big Bang, matter existed as a trillion-degree plasma of quarks and gluons—subatomic particles propagating at relativistic speeds. This primordial state lasted for only a few microseconds before the particles coalesced into more stable hadrons, such as protons and neutrons.
To study this primordial fluid, scientists recreate quark-gluon plasma in large particle accelerators, where heavy ions are accelerated and collided at near-light speeds. These collisions create droplets of plasma roughly the size of an atomic nucleus that exist for only tens of yoctoseconds (\(10^{-24}\) seconds).
One way scientists study this plasma is by tracking other particles, such as heavy quarks, that are not part of the plasma but happen to move through it. The way a heavy particle moves through a fluid reveals information about the microscopic structure of the fluid itself. This general relationship was established by Albert Einstein in 1905.
However, in such extreme conditions where particles are moving at close to the speed of light, Einstein’s classical framework becomes insufficient; there are quantum and relativistic interactions within the plasma that the theory cannot fully capture.
Now, physicists at the Massachusetts Institute of Technology (MIT) and the University of California, Santa Barbara (UCSB) have taken up where Einstein’s theory left off. They have developed a theory predicting how a relativistic particle moving at close to the speed of light experiences momentum shifts and drag forces as it passes through a fluid.
The researchers anticipate that the new theory could be applied to predict how a heavy quark will move through quark-gluon plasma. Such predictions could then be compared with experiments to characterize the microscopic structure of quark-gluon plasma and the type of fluid that must have existed in the universe’s first moments.
“Our result reveals a new source of information carried by heavy quarks in rare, unusually large changes in their motion,” says Jean Du Plessis, a graduate student in MIT’s Center for Theoretical Physics—a Leinweber Institute. “How often those large changes occur depends on the microscopic nature of the plasma, raising the possibility that they could provide a new way to learn what the quark-gluon plasma is actually like inside.”
A Path from Pollen
In 1905, Einstein developed a theory to describe a particle’s motion through fluid and how that motion can reveal the fluid’s microscopic structure. His theory was inspired by the botanist Robert Brown. In 1827, Brown observed a surprising phenomenon: Grains of pollen exhibited erratic, random motion when suspended in seemingly still water. This curious jittering of particles in fluid was later named “Brownian motion.”
Einstein’s 1905 theory showed that Brownian motion was a product of invisible, microscopic forces within the fluid. A particle’s motion is a balance between the drag that the fluid exerts on the particle to slow it down and the stochastic collisions within the fluid that keep the particle moving. These microscopic interactions, Einstein argued, must come from microscopic forces within the fluid, exerted by the atoms and molecules that make up the fluid. The energy of these microscopic particles can add up to jostle larger particles.
Since he first published his theory, scientists have applied Einstein’s explanation of Brownian motion to characterize a host of fluids, including quark-gluon plasma. Like most other fluids, it was assumed that a particle moving through quark-gluon plasma should experience a similar balance of stochastic collisions as a grain of pollen moving through water.
This balance is represented by a bell-shaped probability distribution, where a particle is most likely to experience medium-force microscopic interactions (the highest point of the bell) and an equally low probability of the most extreme interactions (the tail ends of the bell) as the particle moves through and eventually settles into a fluid.
“Einstein showed that the drag and the random kicks must balance in just the right way for a particle eventually to come to the same temperature as its surroundings,” Du Plessis explains. “For the types of fluids he was thinking about, he was correct. But for particles that are moving at relativistic velocities, this relationship breaks down. And we’ve known for twenty years that something is missing. So we have to go beyond the work that Einstein did.”
Kick Speed
In their new study, Du Plessis and Scheihing-Hitschfeld calculated the forces that a heavy quark would likely experience if it flew through a fluid at relativistic speeds, close to the speed of light. They did so by carrying out calculations of the strong nuclear force, which is the fundamental force that pulls together quarks and gluons.
The team started with a scenario of a relativistic heavy quark flying through a gas of weakly interacting quarks and gluons. The weak interactions made this a manageable starting point for their calculations.
For this starting scenario, they calculated the heavy quark’s strong nuclear force and how much its momentum is likely to change as it moves through the gas of quarks and gluons. This calculation produced a curve, or a probability distribution of the quark’s changes in momentum. The same curve can represent the quark’s probability of undergoing stochastic collisions with various forces; stronger microscopic interactions cause bigger changes to the quark’s momentum.
The curve that the researchers calculated was bell-shaped, similar to what Einstein found with other fluids, only for small momentum transfers. And, unlike Einstein’s bell-shaped curve, what the team found showed a tilt for heavy quarks moving at relativistic speeds. The tails at either end of the curve were uneven, revealing an asymmetry in the momentum transfers that a heavy quark experiences.
“The usual Brownian picture assumes that large kicks become exceedingly rare,” Du Plessis says. “For heavy quarks, however, these rare kicks are more common than the bell curve predicts, and are not equally likely in opposite directions: A large kick that slows the quark down more than expected from drag is more likely than an equally large kick that leaves it moving faster than expected.”
The team carried out the same calculation in plasmas made up of different particles. They found that the same tilted curve applies, regardless of the makeup of the plasma. Furthermore, they found that this same structure was already hiding in an earlier calculation in a strongly coupled plasma, despite the differences in the microscopic physics.
“This is the first time that this seemingly universal structure was realized and really formalized,” Du Plessis says. “This gives a concrete path forward for how to better understand the quark-gluon plasma using heavy quarks.”
In terms of what this means for the universe’s first fluid, Du Plessis says that primordial quark-gluon plasma is likely “more strongly interacting than this Brownian motion way of modeling it would suggest. It’s more like a liquid and less like a gas.”
Funding: This research was supported, in part, by the Office of Nuclear Physics in the U.S. Department of Energy’s Office of Science, the U.S. National Science Foundation, and the Simons Foundation.
Published in journal: Physical Review Letters
Title: Heavy Quark Transport Is Non-Gaussian beyond Leading Logarithm
Authors: Jean F. Du Plessis, and Bruno Scheihing-Hitschfeld
Source/Credit: Massachusetts Institute of Technology | Jennifer Chu
Edited by: Scientific Frontline
Reference Number: phy110526_02