UIC researchers make a new breakthrough in studies of the universe’s most perfect fluid
Introduction
The focus of the experimental program in High Energy Nuclear Physics is to understand the properties of the nuclear matter under extreme temperature conditions, such as those of the early Universe. Scientists use ultra-relativistic collisions of heavy nuclei, such as gold or lead, to recreate and study the hot and dense state of primordial matter called quark-gluon plasma (QGP) that filled the Universe just after the Big Bang.
In those studies, it has been established that this primordial matter is opaque to penetrating probes and behaves as the most perfect fluid known ever seen – exhibiting almost no internal friction. The opaqueness results in energy loss by the energetic partons – the fundamental building blocks of nuclear matter – as they traverse the hot medium. Combining these two properties together, the theorists have long predicted that a fast moving probe in the fluid medium should leave a wake behind, much like the disturbance left by a boat moving through water. But despite nearly two decades of experimental searches, directly observing the wake phenomenon proved exceptionally challenging because of the complexity of the heavy ion collisions.
The UIC High Energy Nuclear Physics group, working with the CMS Collaboration at the Large Hadron Collider at CERN, was able to deliver the first direct observation of this wake effect in QGP using streams of particles called “jets” produced by hard scattered partons in heavy ion collisions. Prof. Olga Evdokimov, leading the UIC group, has commented on the discovery: “This observation is a culmination of a long quest to observe the wake phenomenon that for years remained elusive in the experimental data. Without this experimental confirmation, our understanding of QGP properties could not be complete.” Another team member, Dr. Raghunath Pradhan, said: “Observing and quantifying the QGP diffusion wake opens the door to novel precision studies of the properties and dynamics of quark-gluon plasma and promises new insights into the evolution of the early universe.”
For more details, please see:
CMS Collaboration, “Observation of the jet diffusion wake using dijets in heavy ion collisions”, arXiv:2602.19431, Accepted by Phys. Rev. Lett. as Editor’s suggestion. [DOI: 10.1103/g49y-8cjl]