The findings, published in Physical Review Letters, reveal that high-energy streams of particles lose energy as they travel through the material created by these collisions. This effect, called jet quenching, provides an important clue that quark-gluon plasma has formed.

Scientists have previously observed jet quenching in collisions involving much larger atomic nuclei at RHIC, a U.S. Department of Energy (DOE) Office of Science User Facility that operated at Brookhaven National Laboratory from 2000 until 2026. The same effect has also been detected in oxygen collisions at CERN’s Large Hadron Collider (LHC), the European Particle Physics Laboratory.

What makes the new RHIC findings especially significant is that the oxygen collisions occurred at substantially lower energies. The results establish a new lower boundary for the conditions known to produce quark-gluon plasma in this small collision system, helping scientists narrow down what it takes to create matter resembling that of the infant universe.

“The QGP is believed to be the form of matter that filled the early universe — before the protons and neutrons that make up today’s atoms formed. Understanding its properties is essential for uncovering how the early universe evolved in its first microseconds,” said Li Yi, a professor of physics at Shandong University who oversaw the STAR analysis.

“One of the most powerful ways scientists study the QGP is by identifying where its signatures appear — and just as importantly, where these signatures turn off,” she said.

Rongrong Ma, a STAR physicist at Brookhaven Lab, explained why investigating smaller collisions is so valuable.

“This analysis is pushing the boundaries to an extreme case to help us learn something about this jet quenching phenomenon and the conditions needed to generate a QGP.”

How Scientists Recreate Matter From the Early Universe

RHIC was designed specifically to produce and investigate quark-gluon plasma. Traditionally, researchers have accomplished this by accelerating gold nuclei to enormous speeds and crashing them into one another.

Each gold nucleus contains 79 protons and 118 neutrons. When two such nuclei collide with sufficient energy, the boundaries of their protons and neutrons effectively break down, releasing the quarks and gluons inside them. The tremendous energy of the impact also generates thousands of additional quarks and gluons through the conversion of energy into matter, as described by Einstein’s famous equation, E=mc².

The result is a fleeting droplet of primordial matter. It is smaller than an atomic nucleus and survives for less than a billionth of a billionth of a second. Almost immediately, the quarks and gluons recombine into more familiar particles.

Because this exotic substance disappears so quickly, scientists cannot examine it directly. Instead, they use enormous instruments such as the STAR detector to analyze the particles that emerge from the collision.

One particularly revealing clue comes from particles that escape with less energy than expected. Their reduced energy suggests they interacted with the dense mixture of quarks and gluons before escaping.

Researchers pay special attention to jets, which are narrow sprays of extremely energetic particles. These often originate when a quark or gluon from one nucleus strikes a quark or gluon from another, sending two particle jets traveling in opposite directions.

If these paired jets, known as “di-jets,” form near the edge of the collision region, one may have to travel farther through the plasma than the other. Comparing their behavior allows scientists to investigate how the surrounding material drains energy from particles passing through it.

Yi offered a familiar comparison to explain the process.

“You can think of it like shooting a bullet through, say, water,” Yi said. “Because of the resistance, the bullet will slow down. It’s the same thing with these high-energy particles. When they go through the QGP, they will lose energy.”

The Telltale Signs of Quark-Gluon Plasma

For decades, RHIC experiments have provided clear evidence of jet quenching in collisions between gold nuclei. The effect is especially noticeable in jets that travel longer distances through the newly created matter.

Scientists believe these particles lose more energy because they encounter more quarks and gluons before escaping. Jets originating closer to the outer boundary generally pass through less material and experience fewer interactions.

These measurements helped establish that RHIC was successfully producing quark-gluon plasma. Additional evidence came from several other experiments, including an unexpected discovery about how the resulting matter behaves.

Scientists observed that relatively low-energy particles emerging from gold collisions moved collectively, following coordinated patterns. This behavior indicated that the plasma flowed almost like a perfect liquid, with extraordinarily little viscosity or internal resistance.

The discovery was a major surprise when researchers first identified it. Collective flow has since become another important “smoking gun” signature of quark-gluon plasma.

Can Tiny Atomic Collisions Create Primordial Matter?

After establishing these signatures in collisions involving large nuclei, physicists began finding similar collective motion in much smaller systems.

Experiments at the LHC, followed by studies at RHIC, detected collective flow when tiny projectiles such as protons, deuterons, and helium-3 nuclei struck much larger gold or lead nuclei. Researchers at the LHC even observed comparable flow patterns in collisions between individual protons.

These results raised a major question. Did the coordinated particle motion mean that these much smaller collisions were also producing quark-gluon plasma, or could another mechanism explain the observations?

Scientists needed a more definitive sign.

“The next natural question is: Can you see jet quenching, the other key signature that QGP is formed, in these very small collision systems?” asked Brookhaven’s Ma. “We haven’t seen any unambiguous signal, so it’s still kind of under debate in these small systems. We want to search for the smallest system where we can still see an unambiguous jet-quenching signal,” he said.

To investigate, researchers turned to collisions between oxygen nuclei.

An oxygen nucleus contains just eight protons and eight neutrons, making it far smaller than the gold nuclei typically used in RHIC experiments. Oxygen nuclei also have a relatively symmetrical, spherical shape, which makes the geometry of their collisions easier to analyze.

“That means we can more easily look for signs of asymmetric quenching of di-jets produced close to the edge of the circular interaction region in head-on collisions,” said Maowu Nie, an associate professor at Shandong University who contributed to the research.

The scientists could also examine how the outcome changed depending on whether the oxygen nuclei collided directly or merely grazed one another.

“We can also compare how jets behave in head-on central collisions — those that produce the most particles and therefore have the highest likelihood of making a QGP — with collisions where the two oxygen nuclei barely strike each other, which aren’t expected to generate a QGP.”

Nie explained that these grazing collisions are recognizable because they generate far fewer particles than direct impacts.

Oxygen Collisions Reveal a 20% Suppression

To search for jet quenching, the researchers developed an analysis that identifies concentrated streams of high-energy particles and measures how many emerge above a selected energy threshold.

“Our measurements are essentially counting experiments,” said Sijie Zhang, who conducted the research for her master’s thesis under Yi and Nie at Shandong University and is now pursuing a Ph.D. at Yale University.

“We have an algorithm that finds the collimated groups of high-energy particles, the jets, and we count how many of these particles above a certain energy threshold come out.”

Yang He, a former Ph.D. student at Shandong University who developed the analysis framework and now studies oxygen collisions at the LHC, described how the team used different collision types to identify energy loss.

“We compare what we see in back-to-back jets produced near the edge of the system, and we compare what we see in head-on collisions with the counts in non-central collisions where we don’t expect to see quenching.”

The team analyzed STAR measurements from oxygen collisions conducted at RHIC in 2021, at a collision energy of 200 billion electron volts (GeV).

The results revealed a 20% suppression of high-energy particles associated with jet quenching.

Although this suppression was smaller than what physicists typically measure in gold collisions, that difference was expected because oxygen collisions generate much smaller systems.

Crucially, the observed effect was still strong enough to provide clear evidence that quark-gluon plasma had formed.

Comparing Big Bang Matter at Different Energies

Oxygen offers scientists another important advantage. Apart from proton-proton collisions, it is the only nuclear species that has been investigated in collisions at both RHIC and the LHC.

That gives researchers an unusual opportunity to make an “apples-to-apples” comparison between two facilities operating at very different collision energies.

The LHC carried out its first oxygen collisions in 2025. According to results reported by CERN, those experiments revealed both collective flow and jet quenching, providing additional indications that quark-gluon plasma can form in relatively small nuclear collisions.

However, the LHC operated at collision energies exceeding 5,000 GeV, far above the 200 GeV used in the RHIC measurements.

“With a higher energy collision, it’s more likely to create more and more quarks and gluons and a hotter, denser QGP system,” Yi said. “That means the jet quenching signal, if it’s there, should be bigger at LHC compared to RHIC.”

While higher energies make it easier to create the hot, dense conditions needed for quark-gluon plasma, lower-energy experiments may be more useful for determining precisely when those conditions are no longer sufficient.

By reducing the collision energy and using smaller nuclei, physicists can approach the threshold at which the distinctive signatures of primordial matter disappear.

“We are pushing the boundaries in two ways to explore the minimal conditions for creating a QGP, both in terms of the nucleus size by looking at these small oxygen-oxygen collisions, but also by looking at data from lower collision energies — and we still see this crucial QGP signature. That’s a unique feature of our measurement,” Ma said.

New Experiments Could Reveal the Limits of Primordial Matter

Although RHIC has now completed its operations, scientists still have additional oxygen collision data to investigate.

During the collider’s final experimental run, researchers collected measurements using both STAR and sPHENIX, RHIC’s newest detector. The facility concluded its operations with oxygen collisions recorded by both instruments on February 6, 2026.

These remaining datasets could provide further insights into how small a collision system can become, and how low its energy can fall, before quark-gluon plasma can no longer form.

Understanding those limits may help physicists piece together the behavior of matter during the first microseconds of the universe, when quarks and gluons had not yet combined into the protons and neutrons that make up ordinary atoms today.

The research received financial support from the DOE Office of Science, the U.S. National Science Foundation (NSF), and additional international agencies and organizations identified in the scientific publication.

The researchers also used the Open Science Grid, which receives direct support from NSF, along with computing resources at Brookhaven Lab’s Scientific Data and Computing Facilities and the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility located at DOE’s Lawrence Berkeley National Laboratory.

Scientists led by UCL (University College London) found that crocodiles and their ancient relatives spent much of their 250 million years of evolutionary history following warm climates rather than adapting to dramatically different temperatures. When Earth grew hotter, they expanded into new regions, sometimes reaching as far as Antarctica. As temperatures fell, they moved back toward the tropics.

Published in Science Advances, the study analyzed more than 7,000 fossils and records from over 400 living and extinct species. Researchers combined this evidence with computer models that reconstructed Earth’s climate and geography across 245 million years.

The results suggest that tropical and subtropical regions remained suitable for crocodile relatives after the asteroid struck, even as the environments available to many dinosaurs deteriorated. Only the most extreme climate simulations eliminated these potential refuges.

Why Crocodiles Survived the Dinosaur Extinction

The researchers examined how conditions on Earth may have changed following the asteroid impact 66 million years ago. Their simulations indicated that areas near the equator could have remained warm enough to support crocodile relatives, providing places where surviving populations could persist and eventually recover.

Previous modeling by members of the same research team found that suitable habitats for non-avian dinosaurs (i.e., the ones that went extinct) contracted much more severely after the impact.

Lead author Dr. Alfio Alessandro Chiarenza, based at UCL Earth Sciences, said: “The asteroid did not make the whole planet equally hostile to every group.

“Crocodiles’ long-standing preference for warm environments may have placed survivors exactly where suitable climates held up best, like around the equator. These refuges could have sheltered populations that later recovered and radiated out.”

The discovery offers a possible explanation for how a group of animals that appeared around the same time as dinosaurs managed to endure two of Earth’s five major mass extinctions.

Ancient Crocodiles Were Far More Diverse

Modern crocodiles and their close relatives share many recognizable features, but their evolutionary history reveals a much wider variety of body shapes, sizes, and lifestyles.

The broader crocodile lineage, known as pseudosuchians, once included heavily armored plant eaters, predators that hunted entirely on land, and enormous animals weighing more than a metric ton.

Among the most remarkable was Sarcosuchus, a giant predator nicknamed the “super croc” that could reach 12 meters in length. Another prehistoric relative, Postosuchus, was a land dwelling hunter capable of moving on either two legs or all four.

Dr. Chiarenza said: “Crocodiles are often called living fossils, but their ancestors and relatives changed enormously throughout their history, ranging from giants the size of a bus that could have devoured a T. rex, to small, cute animals with the face of a pug.

“Our study shows that, despite all this diversity, what stayed surprisingly consistent was their relationship with climate.”

Fossils Reveal 245 Million Years of Climate History

To investigate that relationship, the scientists reconstructed the temperature and rainfall preferences of 400 crocodile species and their ancient relatives. They used fossil evidence, evolutionary family trees, and detailed maps of where living crocodile species are found today.

The team then compared these preferences with reconstructions of ancient geography and climate to identify where suitable habitats existed at different points in Earth’s history.

A clear pattern emerged. Crocodile relatives generally followed warmer conditions as the planet’s climate changed, expanding their ranges during hot periods and retreating when temperatures cooled.

About 50 million years ago, during the particularly warm early Eocene, global temperatures were approximately 10°C higher than today, and the polar regions had no permanent ice. Crocodile relatives lived at high latitudes across North America, Europe, and even Antarctica.

Over the following millions of years, global cooling gradually reduced the areas where they could thrive. Their geographic range contracted toward warmer tropical regions.

Two Surprising Exceptions

Although this preference for warm environments remained remarkably consistent, the researchers identified two important exceptions.

During the Cretaceous Thermal Maximum, approximately 94 million years ago, global temperatures climbed as much as 14°C above modern levels. One branch of the crocodile family tree evolved the ability to tolerate hotter conditions than the broader lineage could withstand at other times.

A separate adaptation occurred around 34 million years ago in Australia. As a prolonged cooling period reduced rainfall and dried out the continent, an extinct group of crocodiles adapted to increasingly arid, desert-like environments.

These exceptions show that some crocodile relatives could evolve to handle different climates, even though most continued to follow their preferred warm conditions.

What Happened After the Dinosaur-Killing Asteroid?

The researchers also used climate simulations to investigate the decades and centuries immediately following the Chicxulub asteroid impact, which triggered the extinction of most dinosaurs.

They explored several possible environmental consequences, including sunlight being blocked by dust and ash and rising carbon dioxide levels associated with volcanic eruptions.

When the simulations reduced sunlight by 5% or 10%, warm regions near the equator remained suitable for crocodile relatives. These areas could have provided critical refuges during the environmental disruption.

However, a more severe scenario involving a 15% reduction in sunlight left those regions unsuitable as well. The researchers noted that this extreme level of reduced sunlight is now considered unlikely.

Other simulated changes, including increased atmospheric carbon dioxide, did not alter the availability of suitable climates for crocodiles.

Together, the results suggest that crocodiles may have benefited from living in regions where temperatures remained within their preferred range, even as conditions became much less favorable for many dinosaurs.

The Survival Strategy That Could Now Become a Weakness

The researchers emphasized that favorable temperatures were probably only one reason crocodiles survived the mass extinction.

Other characteristics likely improved their chances, including relatively small body sizes, the ability to live partly in water, flexible diets, and modest energy requirements. As ectothermic (“cold-blooded”) animals, crocodiles need less food to maintain their bodies than many animals that generate their own body heat.

The findings also raise important questions about how crocodiles will respond to modern climate change.

For millions of years, moving into regions with suitable temperatures appears to have been one of their main responses to a changing planet. Today, however, expanding cities, agricultural land, and roads increasingly limit where wildlife can move.

Professor Philip Mannion, based at UCL Earth Sciences, said: “These findings have implications for species today. If crocodiles mostly coped with past climate change by moving towards suitable habitats, today’s species will need room to move as the planet warms, and the loss of suitable areas to roads, farms and cities is making that harder.”

The research was funded by The Royal Society, the Leverhulme Trust, and the National Environment Research Council (NERC).

That challenge extends far beyond maps. Similar calculations are important for transportation systems, communication networks, and even biological structures involving proteins, neurons, or other interconnected components. As these networks grow, determining the shortest routes between all their points becomes increasingly difficult.

Computer scientists call this the All-Pairs Shortest Paths (APSP) problem. It involves finding the shortest distance between every pair of points in a graph. In this context, a ‘graph’ is simply a collection of points (called vertices) joined by connections (called edges). Those points might represent cities, PCs, subway stations, or organs, while the connections could stand for highways, cables, railway lines, or blood vessels.

Why Calculating Shortest Paths Is So Difficult

Determining the exact shortest distances across a massive network can consume enormous amounts of computing time and memory. In densely connected networks, some conventional approaches require what mathematicians call cubic time. That means doubling the number of points can increase the computational workload by roughly eight times.

Because the required computation (work) can grow much faster than the network itself, researchers have spent decades searching for more efficient methods. One promising solution is to use approximation algorithms, which calculate estimates rather than exact answers. The goal is to produce distances that are sufficiently accurate while dramatically reducing the amount of computation required.

An important advance came in 1996, when researchers Dor, Halperin, and Zwick (DHZ) developed a method capable of producing distance estimates within twice the actual shortest distance for sufficiently distant points, while operating in nearly optimal time.

This level of accuracy is known as a “2‑approximation.” For example, if the shortest possible route between two locations is 10 kilometers, an estimate satisfying this guarantee would fall between 10 and 20 kilometers.

Rather than examining every possible route, the DHZ approach uses a relatively small selection of points from the network (referred to as sampled vertices). These strategically selected points serve as shortcuts that help the algorithm estimate distances more efficiently.

A Clever Shortcut With a Hidden Weakness

The approach works particularly well when the points being compared are far apart.

Consider a journey between Delhi and Chennai. Because the cities are separated by a considerable distance, the algorithm can often find a sampled point close to the shortest route. This allows it to calculate an estimate that remains within the promised limit of twice the actual distance.

However, the same strategy becomes less dependable when the two points are close together.

Imagine calculating the shortest route between two neighborhoods in the Mumbai suburbs. The sampled points might be located too far from the most direct connection, forcing the algorithm to estimate a much longer route.

For example, two points might be connected by a path containing just two edges, yet the sampled route could contain five. Because five exceeds twice the true distance of two, the result violates the “at most twice” accuracy guarantee.

This weakness meant that the original algorithm’s mathematical guarantee applied only when the points were sufficiently far apart. For closer connections, the same level of accuracy could not always be assured.

That limitation remained a challenge for nearly three decades.

A New Algorithm Breaks Through a Decades-Old Limit

Now, Dr. Manoj Gupta, an Associate Professor at the Indian Institute of Technology Gandhinagar, has introduced a refinement that extends the accuracy guarantee to points much closer together than earlier methods allowed.

Gupta presented his research at the 66th Annual Symposium on Foundations of Computer Science (FOCS 2025) in a paper titled “Improved 2-Approximate Shortest Paths for close vertex pairs.”

The achievement is particularly significant because the improved algorithm preserves the overall runtime of the earlier approach while expanding the range of distances for which reliable estimates can be guaranteed.

The breakthrough comes from rethinking how points are sampled within the network.

Traditional methods depended on a collection of representative points to estimate distances. Gupta’s approach instead introduces a multi-scale sampling strategy that organizes information about the network at several different levels.

By analyzing connections at multiple scales rather than relying on a single sampling layer, the algorithm can account for shorter routes that previous approaches might overlook.

This more sophisticated sampling technique reduces the minimum distance required for the 2-approximation guarantee to hold, without increasing the algorithm’s overall computational complexity.

Returning to the transportation example, the original DHZ method could efficiently estimate sufficiently long journeys, such as the route between Delhi and Chennai. The improved approach extends that dependable accuracy to much shorter connections (such as Mumbai’s suburbs).

The central achievement is not simply a new way to estimate distances. It is the ability to provide the same mathematical accuracy guarantee across a wider range of distances while maintaining computational efficiency.

Why This Computer Science Breakthrough Matters

Massive networks are fundamental to modern technology. Internet infrastructure, transportation systems, social media platforms, and artificial intelligence applications all depend on analyzing complicated webs of interconnected information.

In many situations, obtaining a perfectly exact distance is less important than producing a dependable estimate quickly. When networks contain enormous numbers of connections, the ability to balance accuracy and speed becomes especially valuable.

Gupta’s research strengthens the mathematical foundations of algorithms designed to handle these challenges. Although the findings are theoretical and do not establish immediate improvements to specific commercial systems, they offer a better way to approach distance calculations in large networks.

The work also provides insight into how the overall structure of a network can be understood through carefully selected local connections. By improving how these relationships are sampled and analyzed, researchers can move closer to efficiently estimating distances between every pair of points in a massive system.

Progress in theoretical computer science often comes through incremental refinements rather than dramatic changes. Improving an influential result that dates back to 1996 therefore represents a meaningful advance.

By extending a longstanding accuracy guarantee without increasing overall runtime, the new algorithm pushes beyond an important mathematical boundary and strengthens the techniques underlying some of today’s most complex connected systems.