Colliding gold atoms reveal an unexpected dip in primordial matter fluctuations

A sudden drop in particle momentum variations may pinpoint the critical threshold where primordial quark soup condensed into the building blocks of everyday atoms.

In the earliest fractions of a second after the cosmic expansion began, the universe was filled with an intensely hot fluid rather than distinct atoms.¹ Ordinary matter did not yet exist. Protons and neutrons had dissolved into their constituent parts, forming a dense broth of quarks and force-carrying particles called gluons.¹ As space expanded and cooled, this molten state condensed into the subatomic particles that make up modern stars, planets, and living beings.¹ For decades, physicists have sought to understand exactly how that condensation unfolded, asking whether the cosmic fluid shifted gradually into solid particles or underwent a sharp, violent change akin to water snapping into ice.

Mapping that ancient transition requires creating conditions that mimic the infancy of the cosmos. By driving heavy atomic nuclei into one another at velocities approaching the speed of light, researchers can momentarily recreate droplets of that primordial mixture in laboratory detectors.¹ The behavior of the debris spraying from those microscopic fireballs provides the only direct empirical glimpse into the physical rulebook that governed matter under extreme thermal conditions. Pinning down those rules reveals not only how the cosmos evolved in its first microseconds, but also how matter behaves inside the crushed cores of dead stars.¹

How does matter behave under extreme temperatures?

At extreme thermal energies, the strong nuclear force that binds quarks together weakens enough to let those subatomic particles wander freely as a plasma.¹ Protons and neutrons are composite objects constructed from quarks, which are glued together by force-carrying bosons known as gluons.¹ When nuclear material is heated or compressed past a certain threshold, the boundaries between individual protons break down.¹ The result is a molten state called a quark-gluon plasma, which physicists believe enveloped the entire universe during its first microseconds.¹

Understanding the transition between this plasma and ordinary matter requires charting the equation of state of nuclear matter, a thermodynamic relationship that describes how temperature, pressure, and density interact.¹ In common substances such as water, the equation of state dictates precise points of freezing, boiling, and expansion.¹ For nuclear matter, this relationship defines the fundamental behavior of matter under the most intense conditions found in nature, including the dense interior of neutron stars.¹

When atomic nuclei collide at relativistic velocities, their kinetic energy transforms into a microscopic, superheated fireball.¹ Inside that fireball, protons and neutrons dissolve into a droplet of quark-gluon plasma.¹ As the droplet expands into surrounding vacuum, it cools rapidly. Once its temperature falls below a critical threshold, the free quarks rebind into stable hadrons, including protons, pions, and neutrons, which spray outward in all directions.¹ How forcefully those newborn particles fly sideways reflects the thermodynamic state and internal pressure of the fireball at the instant it solidified.¹ Measuring fluctuations in that sideways spray from collision to collision allows physicists to probe the underlying structure of the phase change.¹

What did the collision measurements reveal?

Physicists detected an unexpected dip in particle momentum variations across collisions of gold nuclei, breaking what was expected to be a completely smooth trend.¹ The measurements were carried out by the STAR experiment at the Relativistic Heavy Ion Collider, an accelerator facility operated at Brookhaven National Laboratory in Upton, New York.¹ The research team reported its findings in Physical Review Letters on September 22, based on the analysis of roughly 1 billion collisions of gold nuclei.¹

To explore different densities of nuclear matter, the collider operated across a spectrum of lower collision energies.¹ Lower collision energies squeeze the colliding nuclei into more compact, denser fireballs than higher-energy impacts produce.¹ For the lowest-energy runs, the accelerator steered a beam of gold nuclei directly into a stationary target, consisting of a thin gold foil placed inside the detector, rather than colliding two opposing beams.¹ This fixed-target configuration allowed the researchers to generate the densest nuclear fireballs achievable at the facility.¹

Particle physicists measure collision energies in giga-electron volts, or GeV, where one GeV corresponds roughly to the energy locked in the rest mass of a single proton.¹ The team focused on collision energies between 3 and 7.7 GeV per colliding pair of protons or neutrons, a regime situated at the lower threshold of the collider's full 200 GeV operating range.¹ In each collision, the detector tracked the transverse momentum of charged particles, which gauges how forcefully debris was kicked sideways out of the central fireball.¹

The researchers then calculated transverse momentum correlations between pairs of emitted particles.¹ These correlations measure whether two particles emerging from the same impact tend to share higher or lower sideways kicks than the average across many collisions. Because temperature fluctuations and fluid expansion affect all particles in a given fireball simultaneously, these pair correlations reflect how much the temperature and collective flow of the fireball fluctuate from one collision to the next.¹

Theory predicts that as nuclear matter nears a thermodynamic critical point, its heat capacity rises sharply.¹ A higher heat capacity means the fireball requires substantially more energy to change its temperature, dampening temperature swings and causing momentum correlations to weaken.¹ When the team analyzed the most direct, head-on collisions, the momentum correlations did not follow a smooth trajectory across rising collision energies.¹ Instead, the correlations dipped noticeably before climbing again.¹

The detected dip departs from the smooth trend established by earlier measurements at higher collision energies with a statistical significance of 5 sigma.¹ In particle physics, a 5 sigma signal satisfies the standard threshold for a genuine physical effect, signifying that random statistical fluctuations in smooth data would produce an equivalent anomaly only about once in 3.5 million experiments.¹ A standard computational simulation of heavy-ion collisions that omitted a critical point succeeded in replicating the broader trend, but completely failed to generate the dip.¹

Why is a nuclear critical point difficult to confirm?

The observed dip in particle variations provides a compelling hint of a phase transition, but alternative physical processes during collisions can also alter momentum fluctuations.¹ The researchers emphasized that the measurement constitutes an intriguing signal rather than definitive proof of a critical point.¹ Heavy-ion collisions are violent, rapid events that endure for merely a few zeptoseconds, making it challenging to separate genuine thermodynamic equilibrium effects from non-critical dynamical phenomena. The authors noted that secondary collision dynamics unrelated to phase boundaries can influence particle fluctuations, and the extent to which those background effects account for the dip remains unresolved.¹ Furthermore, off-center collisions in the dataset exhibited only a faint hint of the dip, which was too weak to serve as independent evidence on its own.¹

The finding represents a single observational anomaly derived from momentum correlations at an experimental collider facility, meaning it cannot stand alone as confirmation of a new phase landmark. Theorists have long posited that nuclear matter possesses a critical point analogous to the point in water where the boundary between liquid and vapor vanishes.¹ At low densities and high temperatures, matter is thought to transition continuously and smoothly into plasma, whereas at high densities, that transition is expected to turn into an abrupt, first-order jump.¹ A critical point marks the precise junction where that smooth crossover gives way to a sharp boundary, but establishing whether this particular momentum dip marks that junction requires reproducing the signal across different physical observables.¹

What questions remain for the early universe?

Mapping the thermodynamic rulebook of dense nuclear matter will require comparing the momentum dip against other observable markers, such as fluctuations in proton production counts.¹ Study co-author Rutik Manikandhan, a postdoctoral physics scholar at The Ohio State University, noted that the research team plans to use the momentum correlation measurements to extract the specific heat of the hot matter.¹ Those experimental values can then be directly compared against supercomputer simulations that calculate quark and gluon interactions from first principles.¹

Establishing whether this dip reflects a true critical point holds broad implications for understanding how ordinary matter stabilized during the Big Bang. In the earliest microseconds of cosmic history, the expanding universe cooled through this very transition, allowing quarks and gluons to coalesce into the protons and neutrons that populate the modern periodic table.¹ Mapping the exact path of that transition explains how matter organized itself across cosmic time.

The same thermodynamic properties also dictate the physical state of the most extreme objects in the contemporary universe. Inside the cores of neutron stars, gravitational compression packs matter to densities that rival those achieved during fixed-target collider runs.¹ Determining whether dense nuclear matter undergoes an abrupt phase transition provides astrophysicists with the equations needed to model stellar collapse, neutron star mergers, and the upper limits of stellar mass.

For the researchers at Brookhaven National Laboratory, the immediate next step involves testing the momentum data against alternative theoretical frameworks and combining it with measurements of proton number fluctuations. Manikandhan indicated that only when several distinct physical measurements converge on the same set of critical conditions can physicists state with confidence that the long-sought boundary of nuclear matter has been located.¹

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