Physicists are uncovering when nature’s strongest force falters

We are getting closer to understanding when the powerful nuclear force releases its grip on the most fundamental constituents of matter, letting the quarks and gluons contained within the particles abruptly transform into a hot soup of particles.

There is a particular combination of temperature and pressure at which all three phases of water – liquid, ice and vapor – exist simultaneously. For decades, researchers have been searching for a similar “tipping point” for matter governed by the strong nuclear force, which binds quarks and gluons into protons and neutrons.

Crushing ions in particle colliders can create a state in which the powerful force breaks down and allows quarks and gluons to form a fluffy “quark-gluon plasma.” But it remains difficult to know whether this transition will be preceded by a critical point. Xin Dong of the Lawrence Berkeley National Laboratory in California and his colleagues are now close to clarifying the situation.

They analyzed the number and distribution of particles created when two very energetic gold ions collided at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York. Dong says they were actually trying to create a phase diagram for quarks and gluons – a map showing what types of matter the strong force allows to form under different circumstances. The new experiment did not definitively identify the critical point on this map, but it significantly narrowed the region where it could be found.

There is a part of the phase diagram where matter gradually “melts” into plasma, like butter softening on the counter, but the critical point would align with a steeper transition, like chunks of ice suddenly appearing in liquid water, says Agnieszka Sorensen of the Michigan Rare Isotope Beam Facility, who was not involved in the work. The new experiment will not only serve as a guide for where to look for this dot, but it also revealed which particle properties may offer the best clues to its existence, she says.

Claudia Ratti of the University of Houston in Texas says many researchers have been eagerly awaiting the new analysis because it gave a precision that previous measurements couldn’t achieve, and this for a part of the phase diagram where theoretical calculations are notoriously difficult. Recently, several predictions about the location of the critical point have converged, and the challenge for experimentalists will be to analyze the data for even lower collision energies corresponding to these predictions, she says.

Unambiguous detection of the critical point would be a generational breakthrough, Dong believes. This is partly because the strong force is the only fundamental force that physicists suspect has a critical point. Additionally, this force played an important role in the formation of our universe: it governed the properties of hot, dense matter created just after the big bang, and it still dictates the 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.