Physicists using the Relativistic Heavy Ion Collider smashed gold nuclei together at near light speed and observed an unexpected pattern in the particles streaming from the impacts. If borne out, the pattern could illuminate how an early-universe quark-gluon plasma cooled into the protons and neutrons that form ordinary matter.
In each collision, particles are ejected sideways. The average strength of that sideways kick varies slightly from event to event. Researchers expected those variations to change smoothly as they tuned the collision energy. Instead, the size of the variations dipped, then rose again. According to the team, the nonmonotonic behavior could signal a long-predicted critical point that marks a change in how nuclear matter transforms between phases.
The STAR experiment at Brookhaven National Laboratory reported that the signal is strong enough to be unlikely as a statistical fluke, while cautioning that it remains a suggestive hint rather than definitive proof. The findings were published Sept. 22 in Physical Review Letters.
A rulebook for extreme matter at the Relativistic Heavy Ion Collider
Protons and neutrons are built from quarks bound by the strong force, carried by gluons. When sufficiently hot or dense, these particles dissolve into a quark-gluon plasma, thought to have filled the cosmos in the first instants after the Big Bang.
Physicists aim to map the equation of state that governs such matter. As one researcher explained, for water the equation of state links pressure, temperature, and density, indicating when it freezes or boils. For nuclear matter, it is the rulebook for behavior under the most extreme conditions, including those in neutron star cores.
The critical point would be a landmark in that rulebook. For water, it is where the boundary between liquid and vapor vanishes. Theorists suspect nuclear matter has an analogous point. At very high temperatures, the transition to quark-gluon plasma may be smooth, while at higher densities it could become abrupt. Some recent calculations place a potential critical point within reach of RHIC’s lower-energy collisions, though this remains unproven.
The work also bears on cosmology. Because the matter created in these collisions resembles the universe a few microseconds after the Big Bang, mapping its behavior could clarify how it evolved into the familiar protons and neutrons of today.
A dip where a smooth trend was expected
To probe this region, the team varied RHIC’s collision energies. Lower energies compress the matter more tightly. For the lowest-energy runs, STAR used a fixed-target configuration in which a beam of gold nuclei strikes a thin gold foil placed inside the detector, producing the densest matter achievable at the facility.
Collision energies are measured in electron volts, typically billions of electron volts, or GeV. One GeV is roughly the energy equivalent of a proton’s mass. The analysis covered about 1 billion collisions between 3 and 7.7 GeV per nucleon pair, the low end of RHIC’s range, which reaches up to 200 GeV.
The team measured each event’s average sideways kick, or transverse momentum, for charged particles, then studied whether pairs of particles from the same collision tended to be kicked harder or more gently than average together. Such correlations reveal global properties of the fireball. If it is slightly hotter or expands more strongly, particles receive an added sideways push in concert.
Near a critical point, the heat capacity is expected to increase sharply, making the temperature less responsive to energy input. That would weaken the correlations. As the collision energy varied, the researchers indeed found a non-smooth change in the most head-on collisions, with a clear dip departing from a smooth trend extrapolated from higher energies. The deviation reached a statistical significance of 5 sigma, the conventional threshold for treating a signal as real.
A commonly used simulation that does not include a critical point reproduced the overall behavior but not the dip. Off-center collisions showed only a faint suggestion of the same feature, which the team said is too weak to stand as independent evidence.
Not the final word yet
The researchers emphasized that the observation is suggestive, not proof of a critical point. Other effects can influence the fluctuations, and their contribution to the dip is not yet fully understood.
Even so, the feature highlights a set of conditions under which nuclear matter appears to change its behavior and provides a precise benchmark for theorists. The next step is to use the correlations to extract the specific heat of the hot matter and compare it with first-principles supercomputer calculations of quark-gluon dynamics.
The team also plans to confront the result with additional theoretical models and to combine it with other observables, including event-by-event fluctuations in the number of protons produced. Only with consistent evidence from multiple measurements, they said, can the field establish whether a critical point exists.