Physicists smashed gold nuclei at nearly the speed of light and found an unexpected pattern in the particles these collisions threw up. If confirmed, that pattern could help reveal how the hot soup of quarks and gluons that filled the universe in the first microseconds after the Big Bang cooled and condensed into the protons and neutrons that make up ordinary matter today. In each collision, the particles are thrown to the sides; The force with which they are thrown, on average, varies slightly from collision to collision. The physicists expected the size of these variations to change smoothly as they adjusted the collision energy. Instead, the variations decreased, shrank, and then grew again. This drop could be a sign of a long-sought “critical point,” a special set of conditions in which nuclear matter changes the way it transforms from one form to another. The signal, detected by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York, is strong enough to make it highly unlikely to be a statistical accident. However, researchers cautioned that the drop is a tantalizing hint, not proof of the long-sought transition. They published their findings on September 22 in the journal Physical Review Letters. Latest Videos from Live Science A rulebook for extreme matter Protons and neutrons are made of smaller particles called quarks, which are held together by the “strong force” carried by particles called gluons. When heated or squeezed enough, protons and neutrons fuse into a hot soup of free quarks and gluons known as quark-gluon plasma. This primordial soup of particles is believed to have filled the universe in the first milliseconds after the Big Bang. Physicists want to specify how this soup occurred, measuring the so-called equation of state of nuclear matter. “In the case of water, it tells how pressure, temperature, and density are related, and therefore when it freezes, boils, or expands,” study co-author Rutik Manikandhan, a postdoctoral researcher in physics at The Ohio State University, told LiveScience in an email. For nuclear matter, “it’s the basic rulebook for matter in nature’s most extreme conditions, like the nuclei of neutron stars,” Manikandhan added. You may be interested The “critical point” is a key milestone in that rule book. “For water, it is the point where the boundary between liquid and vapor disappears,” Manikandhan said, and theorists have long suspected that nuclear matter has a similar point. At extremely high temperatures, matter melts gently and gradually to form plasma of quarks and gluons, but at higher densities the change can become abrupt. The critical point would mark where one type of transition becomes another. Some recent calculations place this point within reach of the RHIC’s lowest energy collisions. “But all this is still a conjecture and there is nothing concrete yet, neither on the part of the experimentalists nor the theorists,” Manikandhan said. Get the world’s most fascinating discoveries delivered straight to your inbox. The work is also important for cosmology. “Because the matter we create in these collisions looks like the matter that filled the universe just a few microseconds after the Big Bang, mapping how it behaves helps us understand how the universe evolved from a hot soup of quarks and gluons to the protons and neutrons that make up everything today,” Manikandhan said. The central part of the three-story STAR detector at the Relativistic Heavy Ion Collider (RHIC). Scientists recently used the detector to study conditions in the early universe, just milliseconds after the Big Bang. (Image credit: Brookhaven National Laboratory) A dip where a smooth trend was expected To explore this territory, the researchers ran RHIC at a range of collision energies. The lower the energy, the more tightly the colliding matter was squeezed together. For its lower-energy runs, STAR used a “fixed target” configuration, in which a beam of gold nuclei hits “a thin sheet of gold placed inside the detector,” Manikandhan said, rather than a second oncoming beam. This produces the densest matter that RHIC can produce. Collision energies are measured in electronvolts (the energy an electron gains when it is accelerated by 1 volt). This is a tiny amount, so particle physicists typically work in billions of electron volts or gigaelectron volts (GeV). One GeV is approximately the energy contained in the mass of a single proton, according to Einstein’s famous equation E = mc2. The team analyzed approximately one billion collisions with energies between 3 and 7.7 GeV per pair of colliding protons or neutrons. That’s the lowest point of RHIC’s range, which reaches 200 GeV. What to read next In each collision, the team measured the force with which the charged particles were thrown laterally out of the fireball, a quantity known as transverse momentum. The researchers then looked for correlations between the particles. A correlation measures whether two things tend to change together. Here, the team tested whether pairs of particles from the same collision tended to be thrown harder than average, or both more gently. That reveals something about the fireball as a whole. If a fireball is slightly hotter or expands more strongly, all of its particles get an extra side kick. “Those correlations reflect how much the fireball’s temperature and flow fluctuate,” Manikandhan said. Near a critical point, the heat capacity of matter (the amount of energy needed to raise its temperature) is expected to skyrocket. That makes it harder to change the temperature of the fireball, so the correlations should weaken. “If matter is approaching a critical point or phase change, we would expect to see those correlations change in an unusual, non-smooth way as we vary the energy of the collision,” Manikandhan said. That’s what the team saw in most of the head-on collisions. “Instead of changing smoothly with energy, the correlations show a drop,” Manikandhan said. The researchers compared the data to a smooth trend based on previous STAR measurements at higher energies. The drop deviates from that trend with a statistical significance of 5 sigma, which standard physicists typically require before treating a signal as real. It means that if the true trend were smooth, the random spread in the data would produce such a steep drop only once every 3.5 million trials. “A smooth trend is what you would expect from ordinary nuclear matter, so a dip suggests that something more interesting is happening under those conditions,” Manikandhan said. In contrast, a widely used computer simulation of the collisions, which does not contain any hotspots, reproduced the overall trend but not the decline. The off-center collisions showed only a faint hint of the same feature, which is too weak to count as evidence on its own. The last word has not yet been had. “The result is suggestive, not proof of a critical point,” Manikandhan said. Non-tipping point effects can also shape these fluctuations, and it’s not yet clear how much of the decline they might explain, the researchers noted. Still, the drop “points to a set of conditions where the behavior of nuclear matter changes, and gives theorists a new, precise measurement to test their calculations,” Manikandhan said. Next, the team plans to use the correlations to “extract the specific heat of hot matter,” Manikandhan said. They will then compare it to supercomputer simulations that calculate the behavior of quarks and gluons from first principles. The researchers also plan to test the decay with more theoretical models and combine it with other measurements, such as fluctuations in the number of protons produced in collisions. “Only when the different measurements coincide will we be able to say for sure if there is a critical point,” Manikandhan said. Aboona, B.E., Adam, J., Adamczyk, L., Aggarwal, I., Aggarwal, M.M., Ahammed, Z., Alshammri, A.K., Aschenauer, E.C., Aslam, S., Atchison, J., et al. (2026). Non-monotonicity of transverse momentum correlations in Au + Au collisions in RHIC. Physical Review Letters, 137(13), 132301. https://doi.org/10.1103/2xsn-rgx3
