STAR scientists found an unexpected energy-dependent pattern in particles produced when gold nuclei collided: sideways-momentum correlations in the most central collisions changed nonmonotonically across collision energies from 3.0 to 7.7 GeV per colliding nucleon pair. The result may be sensitive to a predicted feature of matter called the QCD critical point, but it does not establish that the point has been found.
What did the scientists find?
The STAR Collaboration measured correlations in the transverse momentum of charged particles created in fixed-target gold-on-gold collisions at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC). Transverse momentum means momentum directed sideways, across the beam’s path. The researchers examined whether pairs of particles from the same collision tended to have related sideways momentum, then compared that observable across collision energies.
In central collisions—those in which the nuclei collide head-on or nearly so—the correlations varied nonmonotonically with energy: rather than following a smooth trend, the pattern changed in a way that stood out across the measured range. The result is reported in the 2026 STAR study, “Nonmonotonicity of Transverse Momentum Correlations in Au+Au Collisions at RHIC,” published in Physical Review Letters on September 22, 2026.
How strong was the signal?
STAR reported approximately 5σ statistical significance for the nonmonotonic pattern in central collisions. That describes the strength of the observed pattern in the data under the study’s statistical analysis; it is not a 5σ finding that the QCD critical point exists.
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| Comparison | Reported significance | What it indicates |
|---|---|---|
| Central-collision data | Approximately 5σ | The study reports a statistically significant nonmonotonic energy dependence in the measured correlations. |
| Transport-model calculations | Approximately 2σ | The comparison does not support a claim of nonmonotonicity. |
| Mid-central collision data | Approximately 1.4σ | The comparison does not support a claim of nonmonotonicity. |
These are the comparisons reported by the STAR Collaboration in its 2026 paper. Statistical significance for a measured pattern is distinct from evidence that a particular physical explanation for that pattern is correct.
Why might the pattern matter?
The authors say the observations provide new constraints on the equation of state at high baryon density and may be sensitive to the presence of a QCD critical point. The equation of state describes how matter’s properties—such as pressure and density—relate under different conditions. A critical point is a predicted feature in the map of possible states of quantum chromodynamics (QCD), the theory describing quarks and gluons and their strong interactions.
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The collision-energy dependence makes the measurement useful to researchers investigating how strongly interacting matter changes as conditions vary. But the correlation pattern is an observable, while the critical point is a possible interpretation. The paper does not claim that STAR has discovered or confirmed the critical point.
What was collided, and what does RHIC study?
Gold nuclei are the centers of gold atoms, stripped of their electrons. At RHIC, colliding such nuclei briefly creates extremely hot, dense matter in which quarks and gluons can move freely. Brookhaven describes this quark-gluon plasma as behaving more like a liquid than the gas scientists initially expected. That broader result provides context for RHIC’s gold-ion program; it is separate from STAR’s new transverse-momentum-correlation measurement.
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Which collisions and energies were analyzed?
The study used fixed-target Au+Au collisions in the Beam Energy Scan Phase II program. The analyzed range was 3.0–7.7 GeV per colliding nucleon-nucleon pair. Researchers compared central and mid-central collisions, with the reported nonmonotonicity claim supported by the central-collision data rather than the mid-central comparison.
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