Physicists are uncovering when nature’s strongest force falters

We're getting closer to understanding when the strong nuclear force loosens its grip on the most basic constituents of matter, allowing quarks and gluons inside particles to suddenly turn into a hot particle soup.

There is a special combination of temperature and pressure at which all three phases of water – liquid, ice and steam – exist simultaneously. For decades, researchers have been searching for such a “critical point” for matter controlled by strong nuclear forcewhich binds quarks and gluons into protons and neutrons.

The destruction of ions in particle colliders can create a state in which the strong force breaks down and allows quarks and gluons to form a liquid “quark-gluon plasma.” But it remains unclear whether this transition is preceded by a critical point. Xin Dong at Lawrence Berkeley National Laboratory in California and his colleagues are now closer to clarifying this question.

They analyzed the number and distribution of particles produced by the collision of two very energetic gold ions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York State. Dong says they were actually trying to create a phase diagram of quarks and gluons—a map showing what types of matter the strong force allows to form under different circumstances. The new experiment did not identify with great certainty the critical point on this map, but it significantly narrowed the area where it could be located.

There is a part of the phase diagram where a substance gradually “melts” into plasma, like butter softening on a counter, but the critical point coincides with a more abrupt transition, as if chunks of ice suddenly appear in liquid water, he says. Agnieszka Sorensen at the Rare Isotope Beam Facility in Michigan, which was not involved in the work. She says the new experiment will not only provide clues about where to look for this point, but will also show what properties of particles can provide the best clues about its existence.

Claudia Ratti from the University of Houston in Texas say many researchers were eagerly awaiting this new analysis because it gave precision that previous measurements could not achieve, and did so for a part of the phase diagram where theoretical calculations are notoriously difficult. Recently, several predictions for the location of the critical point have converged, and the challenge for experimentalists will be to analyze data for even lower collision energies consistent with those predictions, she says.

Detecting the tipping point unambiguously would be a generational breakthrough, Dong says. This is partly due to the fact that strong power is the only fundamental force which physicists suspect has a critical point. In addition, this force played a significant role in the formation of our Universe: it determined the properties of the hot and dense matter created immediately after the Big Bang, and still determines structure of neutron stars. Dong says collider experiments like the new one could help us understand what's going on inside these exotic cosmic objects once we complete the phase diagram of the strong forces.