Twenty-five years ago, nuclear scientists at Brookhaven National Laboratory using the Relativistic Heavy Ion Collider (RHIC) to collide atomic nuclei at near-light speeds and temperatures more than 100 million times hotter than the Sun succeeded in creating quark-gluon plasma (QGP), a fundamental, extremely hot form of matter that filled the early universe in the first few microseconds of its existence and is believed to be present today in the cores of neutron stars.
In the decades since that breakthrough moment, Texas A&M University physicist and Cyclotron Institute member Dr. Ralf Rapp has been making his own essential contributions in multiple areas of quantum chromodynamics (QCD), a theory of the strong force that describes how the QGP and other forms of nuclear matter interact. Within this extremely hot phase of matter, protons and neutrons are dissolved into their elementary building blocks, quarks and gluons, which are credited with creating more than 98% of the visible mass in the universe. But for Rapp and his fellow nuclear scientists, the big question is how?
“A key goal in modern research is to understand the quark-gluon plasma’s properties from the fundamental interactions of the standard model of elementary particle physics,” Rapp explained. “A pivotal question is how hadrons — bound states of quarks and gluons that make up the world around us — dissolve into their constituent particles.”
In his quest for answers, Rapp has been focused on one long-standing problem: the survival of quarkonia, which are bound states of heavy quark pairs held together by the strong force in the QGP. In recent research led by two of his graduate students, Zhanduo Tang and Biaogang Wu, Rapp and his colleagues have provided the first definitive criterion validating the existence and melting temperatures of quarkonia while also quantifying their masses and dissociation widths within the QGP.
“Quarkonia have long been recognized as unique probes of the quark-gluon plasma produced for a short moment in laboratory experiments,” Rapp said. “We have found that the melting of individual states is realized by the disappearance of a single pole in both the real and imaginary parts of the 𝑇 matrix, accompanied by a decreasing residue before melting. In essence, quarkonia do not immediately dissolve in the quark gluon plasma but can survive as resonances to surprisingly high temperatures.”
For two decades and counting, Rapp and his team, which includes collaborators from Brookhaven as well as Kent State University, have been studying the properties of quarkonia in the QGP — specifically, their so-called spectral functions. To date, they have found solid evidence that their properties are strongly modified due to their interactions with thermal quarks and gluons within the plasma. For instance, he says their spectral line widths strongly broaden, indicating their rapid decays, but not to an extent that would allow his team to make a rigorous determination as to whether or not the bound states are actually dissolved.
“Through discussions in our research group meetings, including those within the HEFTY collaboration, we developed the idea to extend the theoretical calculations to ‘imaginary energies’ — an established method to find bound states in the vacuum where they appear as poles in the complex energy plane,” Rapp said. “Here, we first deployed this method in the quark-gluon plasma and were very surprised that we found a rather rich bound-state spectrum (i.e., poles in the complex-energy plane) that we would never have expected from our results on the real-energy axis alone (i.e., with vanishing imaginary part).”
The team’s findings, which are posted to arXiv and also published in Physical Review Letters, imply that very strong interactions in plasma can support bound states to much higher temperatures than expected and, as a result, call into question widely used schematic criteria based on comparisons of binding energies with decay widths.
“In addition to providing new insights into the structure of the quark-gluon plasma, our studies will have a significant impact on the interpretation of experimental data from high-energy heavy-ion collisions,” Rapp said. “Current efforts are focused on implementing the properties of the quarkonium states into simulations of their transport through the quark-gluon plasma as produced in high-energy heavy-ion collisions at RHIC and the Large Hadron Collider at CERN. Measurements of quarkonia remain a high priority for future experiments at these facilities, providing critical experimental tests of our theoretical predictions to take maximal advantage of the large investments into these campaigns.”
The team’s research is supported by the Texas A&M-led, Department of Energy-funded HEavy-Flavor TheorY (HEFTY) for QCD Matter collaboration, for which Rapp serves as principal investigator, along with Rapp’s single-principal investigator grant from the National Science Foundation.