Physicists collided gold nuclei at very high speeds and discovered an unexpected pattern in the particles released as a result of the impact. This finding provides clues about how the primordial matter formed shortly after the Big Bang cooled down to form the protons and neutrons that constitute the matter of the modern universe.
The anomaly may indicate what is known as the 'critical point' of nuclear matter—a state where changing one state of matter into another alters its behavior, according to the study. The data were recorded by the STAR detector installed at the RHIC accelerator at Brookhaven National Laboratory in New York.
To reproduce conditions similar to those of the early universe, researchers from Brookhaven National Laboratory made gold nuclei collide at various energy levels. The team analyzed about a billion collisions in the range of 3 to 7.7 GeV, which is the lowest energy zone of RHIC.
At lower energy collisions, a fixed target system was used: a beam of gold struck a thin sheet of the same metal. This configuration allows for the production of the densest matter achieved by the accelerator.
After each impact, scientists observed with what intensity charged particles were ejected sideways. This measurement is known to physicists as transverse momentum. They also compared the movement of the particles to check whether they tended to be ejected together with greater or lesser force.
A gradual change was expected as the collision energy varied. However, the result was different: the effect decreased, reached its lowest point, and then began to rise again.
This difference was significant enough to be considered statistically very significant. The result reached 5 sigma, a level that physicists use to indicate that it is unlikely to be a coincidence. Calculations suggest that if the observed change were merely random, such a deviation would occur approximately once in 3.5 million attempts.
The observation helps scientists understand a kind of 'guidance' of matter under extreme conditions, called the equation of state. It describes how characteristics such as temperature, pressure, and density relate to each other.
Protons and neutrons are composed of even smaller particles—quarks. They remain bound by the strong interaction, one of the fundamental forces of nature. Under exposure to extreme temperatures or densities, protons and neutrons can transform into quark-gluon plasma—a kind of 'soup' of particles that is thought to have filled the universe in the first moments after the Big Bang.
"For water, this indicates when it freezes, boils, or expands," explained Rutik Manikandhan, a physics postdoc at the University of Ohio State. According to the researcher, for nuclear matter, the equation of state functions as a set of rules for understanding its behavior under the most extreme conditions of nature.
It is in this scenario that the so-called critical point appears. These are special conditions under which the way matter transitions from one state to another can change. Discovering this region would help physicists better understand how the quark-gluon plasma of the primordial universe turned into the matter we know.
The signal attracted attention because widely used simulations that do not account for the existence of a critical point reproduced the general trend of the data, but not the drop that was observed. However, in off-center collisions, the same effect appeared only weakly.
The researchers remain cautious. Other phenomena may alter the observed variations in particles, and it is yet to be determined how much they explain the registered behavior.
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Manikandhan stated: "This result is indicative, not proof of the critical point."
The team now plans to calculate the specific heat capacity of the hot matter formed during the collisions and compare this result with models describing the behavior of quarks and gluons. Other measurements, such as changes in the number of produced protons, will also be analyzed.
The researcher concluded: "Only when different measurements agree can we confidently assert the existence of a critical point."
If the hypothesis is confirmed, the result could help clarify an important stage of the universe's evolution: the transition from quark-gluon plasma to matter composed of protons and neutrons.