The Roadmap to Quark Soup

primordial soup
Scientists seeking to plot points on the nuclear phase diagram — a map of the different states of nuclear matter and transitions among them under different conditions of temperature and density — have new signposts (curved line) to look for as they search for evidence of a “critical point.” (LHC green oval = matter found by the Large Hadron Collider; RHIC = matter found by the Relativistic Heavy Ion Collider.) Image courtesy of Brookhaven National Laboratory

Did the primordial soup of fundamental particles that filled the early universe suddenly “freeze” to form the protons and neutrons that make up visible matter today? Scientists now have new signposts to look for as they map out that transition from primordial quark-gluon plasma to matter as we know it. The research identifies key patterns. These patterns would be proof of the existence of a “critical point” in the transition among different phases of nuclear matter. At the critical point, matter as we know it today and the particle soup of the early universe are virtually indistinguishable.

The Impact

A detailed knowledge of different phases of water — liquid, solid ice, and steam — teaches us about the force that binds a water molecule. Similarly, identifying various phases of nuclear matter and the “critical point” will provide insight into the strong force that binds nuclear matter. Scientists don’t yet know how this knowledge about the strong force might be applied. But as they point out, they didn’t know how the collective properties of electrons, considered just as exotic when discovered a century ago, would benefit society and industry.


By tracking particles that emerge from nuclear collisions at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science user facility at Brookhaven National Laboratory in New York, physicists are seeking to understand nuclear phase transitions. They want to learn how ordinary nuclei “melt” to create a quark-gluon plasma. The plasma is the stuff that existed in the very early universe before atoms or even protons and neutrons. By understanding how nuclei melt, they can learn how the quarks and gluons that ultimately make up these nuclear particles “freeze out” as they did at the dawn of time. That freezing formed the visible matter of today’s world. They believe that two different types of phase changes can transform the hot quark-gluon plasma into ordinary protons and neutrons. Importantly, they suspect that the type of change depends on the collision energy, which determines the temperatures generated and how many nuclear particles get caught up in the collision. By systematically colliding nuclei at a wide range of energies, physicists in RHIC’s STAR collaboration are exploring the different types of phase changes across the nuclear phase diagram. They are particularly interested in searching for evidence of a “critical point,” where the quarks and gluons that make up protons and neutrons transition to a plasma very quickly — almost as if all the water in a pot turned to steam in a single instant. A new theoretical analysis has shown them exactly what to look for the closer they get to this critical point. Specifically, the analysis predicts patterns in how the properties of particles emitted from the collisions are correlated as the energy of the collisions changes. The theory is an advance because it takes into account the dynamic conditions of the expanding quark-gluon plasma, unlike the more static situation of a pot of boiling water. If the STAR collaboration looks at the data in a particular way and sees the predicted signposts, they can claim without any ambiguity that they have seen a critical point.