“In this work we propose a new way to describe the hadronization of a quark gluon plasma.”

With the advent of our paper the so-called baryon puzzles at the Relativistic Heavy Ion Collider (RHIC) could be solved. At present our paper provides the most comprehensive description of hadron spectra at RHIC so far. It further provides very direct evidence that quark degrees of freedom are present during the collision through a quark counting rule for an observable called elliptic flow. This was met with great excitement, since most signals of the phase transition are only indirect and hard to interpret.

Several groups are working with the same idea and try to refine our understanding of high energy nuclear collisions. Because of its simplicity I expect quark recombination to be a very useful tool for the further analysis of data from RHIC and future experiments, like the Large Hadron Collider at CERN. The first goal of RHIC was to prove that a quark gluon plasma is created. The quark counting rule that can be understood with quark recombination is a major step toward that claim.

In this work we propose a new way to describe the hadronization of a quark gluon plasma. A few microseconds after the Big Bang, the universe was extremely hot. The temperature was roughly (~ 1,000,000,000,000 K), more than 1,000 times hotter than in the interior of the sun. Under such conditions all matter dissolves into the most fundamental building blocks. Even atomic nuclei, protons, and neutrons can no longer exist, but evaporate into quarks and gluons. When the universe expanded and cooled to a certain critical temperature, a phase transition occurred at which the quarks and gluons were freezing out and creating bound states, the known hadrons. The protons and neutrons among these hadrons eventually formed the nuclei of all the chemical elements observed in the universe. The phase transition to the quark gluon plasma is studied at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab on Long Island. This machine smashes gold ions into each other with extreme energies (about 40 TeV per collision). Part of this energy is converted into heat. For a very short time, roughly 10^(-23) seconds, the temperature peaks above the phase transition temperature and a quark gluon plasma is created. We try to study the properties of this high-temperature phase by analyzing the particles created in the collision. Our paper describes the hadronization process through a recombination of quarks into bound states (the hadrons). Several observations at the Relativistic Heavy Ion Collider could not be understood with conventional wisdom, e.g., through an angle of 90 degrees from the beam axis as many protons as pions are seen with large momentum (several GeV/c), while one expected to see a ratio of roughly 1:4. In our paper we explain how protons (consisting of 3 quarks) are boosted to larger momentum by a factor of 3, while pions (consisting of 1 quark and 1 antiquark) are only boosted by a factor of 2, which cancels the inherent suppression of protons.

I first started working on problems related to high-energy nuclear collisions while working on my Ph.D. in Regensburg, Germany. I got involved full-time with RHIC physics during my stay as a postdoc at Duke University, where this paper was written in collaboration with Berndt Muller, Chiho Nonaka, and Steffen A. Bass.