Physicists are uncovering when nature’s strongest force falters

Scientists are advancing their understanding of the conditions under which the strong nuclear force releases its grip on the fundamental building blocks of matter, allowing quarks and gluons to transition into a dynamic state of hot particle soup.

Much like the distinct phase of water where ice, liquid, and vapor coexist at specific temperatures and pressures, physicists have long sought a parallel “critical point” illustrating when matter governed by the strong nuclear force transitions between states, binding quarks and gluons together into protons and neutrons.

High-energy collisions at particle colliders can induce a state where the strong force diminishes, leading to a “quark-gluon plasma” state. However, the existence of a critical point preceding this transition remains unclear. Xin Dong and his colleagues from Lawrence Berkeley National Laboratory have made significant strides in this pursuit.

By examining the particle production dynamics following the collision of two energetic gold ions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York, they sought to map a phase diagram for quarks and gluons, revealing different types of matter the strong force can foster under varied conditions. While the experiment did not conclusively identify the critical point, it markedly narrowed down the likely areas on this phase map.

According to Agnieszka Sorensen from the Facility for Rare Isotope Beams in Michigan, who wasn’t involved in the research, there is a segment of the phase diagram where matter transitions into plasma gradually, akin to butter melting at room temperature, whereas the critical point signifies a more sudden transition, similar to ice forming crisply within liquid water. The findings provide not only a roadmap for where to search for this critical point but also highlight specific particle characteristics that may signal its presence, she adds.

Claudia Ratti from the University of Houston expressed excitement among researchers regarding this analysis, which has achieved precision levels unattainable by previous studies, specifically within a notoriously challenging section of the phase diagram. With various predictions converging on a critical point’s location, the task now rests on experimentalists to scrutinize data from even lower collision energies consistent with these forecasts.

A definitive identification of the critical point would represent a major breakthrough, asserts Dong. The strong force is particularly noteworthy as the only fundamental force suspected to possess such a critical point. Its implications are profound, having influenced the characteristics of dense matter shortly after the Big Bang and continuing to shape the structures of neutron stars. Dong indicates that experiments like the one described could shed light on the inner workings of these extraordinary cosmic entities as the strong force phase diagram is completed.