A theoretical framework based on the general properties of critical phenomena can reveal when quark matter is approaching the critical point between a gas of hadrons and a plasma of quarks and gluons

Critical points are ubiquitous in nature. The most familiar one is the critical point between liquid and gas in water. A critical point between a gas of hadrons and a plasma of quarks and gluons has long been hypothesized to occur in quark matter. Understanding whether this critical point exists remains a central challenge in quantum chromodynamics.

Now Giovanni Sordi at Royal Holloway and André-Marie Tremblay at the University of Sherbrooke have introduced a mathematical tool for revealing when quark matter is approaching this quark-hadron critical point. They showed that key to detecting an incipient critical point is the so called “Widom line”, a crossover line emerging from the critical point, which quantifies the enhancement of fluctuations beyond the critical point.

The concept of the “Widom line”, named after the scientist Benjamin Widom, is based on the general features of phase transitions and is already widely used in the context of classical fluids and of quantum materials. Based on the analogies with these systems, Giovanni Sordi and André-Marie Tremblay suggested a few directions where the concept of the Widom line could bring news ideas in the phase diagram of quantum chromodynamics. 

Article: G. Sordi and A.-M. S. Tremblay, Introducing the concept of the Widom line in the QCD phase diagram, Phys. Rev. D 109, 114020 (2024)