Physicists found strong hints of 'tipping point' in nuclear matter

Physicists at a New York collider have tightened the search for a long-predicted tipping point in nuclear matter. The new result marks a sharp feature at 19.6 billion electron volts per nucleon pair that could signal where the strong force loosens its hold.

The measurements come from the STAR detector at the Relativistic Heavy Ion Collider in Upton, New York. The machine smashes gold ions together and heats matter to roughly 7.2 trillion degrees Fahrenheit (4 trillion degrees Celsius).

Why nuclear matter is important

The team is chasing a special landmark in quantum chromodynamics, the theory of the strong force. At this landmark, matter would switch behavior in a way that changes how particles are born from collisions.

Xin Dong of Lawrence Berkeley National Laboratory (LBL) led the analysis, which narrows where that landmark might sit. The STAR collaboration searched for patterns that would stand out only if a critical point exists.

Physicists picture the nuclear phase diagram, a map linking temperature and density to different kinds of matter. At low heat and density, quarks and gluons hide inside protons and neutrons.

At higher heat or compression, particles can break open and mix into a quark-gluon plasma, a hot liquid of free quarks and gluons. The question is whether the switch from ordinary matter to this liquid can show a sudden turn that matches a critical point.

How scientists searched

STAR looked at the event-by-event spread in the number of protons produced. The key observable is a cumulant, a statistical measure that captures fluctuation patterns beyond simple averages.

One cumulant ratio, called the fourth to second, dipped near 19.6 GeV in the most head-on gold collisions. The collaboration reports that the deviation from noncritical expectations reaches about 2 to 5 standard deviations.

The analysis also checked factorial versions of these metrics. Ratios such as kappa two over kappa one and kappa three over kappa one showed related departures from baselines.

Those extra handles help because different observables respond differently if a critical point is nearby. The pattern across several metrics reduces the chance that a single detector effect mimics a signal.

What results exclude

Earlier hints suggested where a critical point could not be. The latest results strengthen that exclusion at the highest energies.

“BES-II precision measurements rule out the existence of a critical point in the regions of the QCD phase diagram accessed at LHC and top RHIC energies, while still allowing the possibility at lower collision energies,” said Bedangadas Mohanty, who co-led parts of the analysis.

That statement lines up with the new STAR pattern. The dip appears at intermediate energies, not at the very top of RHIC or at the Large Hadron Collider.

A minimum in the kurtosis-related ratio is what many theories predict if a critical point sits nearby. The observed minimum does not fix the point’s exact location, but it narrows the region.

Nuclear matter behaviour is important elsewhere

The strong force shaped the early universe when it was still a few microseconds old. It also helps determine the internal makeup of neutron stars.

Modelers use an equation of state, a relation between pressure and density in matter, to connect lab results with those stars. A U.S. Department of Energy report explains how such relations tie to neutron star sizes and how they deform during mergers.

Knowing if a critical point exists would anchor parts of that relation. It would tell theorists how matter changes as it is compressed and heated.

Heavy ion data also test models used in supernova and merger simulations. Better constraints on fluctuations help pin down the physics those codes need.

What’s next for nuclear matter

To confirm a critical point, experiments must probe even lower energies with equal or better precision. That work includes reanalyzing recent runs and planning targeted scans.

Brookhaven’s Beam Energy Scan program completed a major phase with high statistics runs and key detector upgrades. A Brookhaven update describes how those data enable precision fluctuation measurements across 7.7 to 27 GeV.

The next priority is to analyze the lowest energy sets and to refine transport and hydrodynamic models that include a built-in critical point. Theory must match not only one ratio but the full correlated pattern.

Other facilities may also contribute. Together they can test whether the same fluctuation features emerge under different conditions.

How to verify signals

Critical point signatures can be faked by mundane effects. Careful baselines and controls are therefore essential.

STAR compares central collisions to more peripheral ones, where fewer nucleons interact. That helps remove simple volume effects that would otherwise inflate fluctuations.

The team also folds in acceptance corrections and evaluates backgrounds. Those steps aim to isolate genuine dynamics from detector artifacts.

The wider view

Even a firm exclusion is progress in this search. Each ruled-out region sharpens the map of nuclear matter.

A confirmed critical point would be a reference feature for kurtosis, a measure of how heavy the tails of a distribution are. That reference would guide both lattice calculations and transport models.

The search is also a stress test for collaboration between experiment and theory. Converging predictions and high precision data are starting to meet in the same patch of the phase diagram.

The study is published in Physical Review Letters.

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