ELEMENTARY PARTICLE PHYSICS

Researchers studying subatomic particles that govern our world have long been interested in describing phenomena that happen to the constituents of matter called protons and neutrons, called quarks and gluons, under extreme conditions. Using GCS computing resources, scientists were able to use quantum chromodynamics simulations to reveal that exotic “strange” and “charm” quarks freeze out at roughly the same temperature as the light quarks. In addition, it was found that more strange and charmed bound states should exist than have been detected experimentally thus far. These results are helping to interpret prove experimental observations at CERN’s Large Hadron Collider and Brookhaven National Laboratory’s Relativistic Heavy Ion Collider.

Atomic nuclei consist of electrically neutral neutrons and positively charged protons. Although the latter should repel one another, they are nevertheless bound in the nuclei. The reason for this are the strong interactions. Numerous experiments have shown that the underlying force actually is the interaction between the constituent, subatomic particles of protons and neutrons, the quarks, and the equally subatomic particles, the gluons, which bind them. The theory describing this interaction is called Quantum Chromodynamics, or QCD.

While quarks and gluons are, at room temperature, confined within protons, neutrons and other bound states, their behavior changes drastically when heated to temperatures of about 100,000 times the interior of our sun, or compressed to something like 40 billion tons per cubic centimeter. In collisions of heavy nuclei, such as lead or gold, form a new state of matter at sufficiently high collision energy, called the Quark Gluon Plasma (QGP). The necessary energy can only be provided by the largest accelerators to date, the Large Hadron Collider (LHC) at CERN and Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.

These colliders create, for a short moment, a hot and dense QGP fireball which then expands and correspondingly cools down. At some point the composition of the fireball freezes-out into normal quark bound states which are then collected and analyzed by the detectors installed. From the analysis of the debris of such a collision it is to be deduced what the properties of the QGP are, or rather have been a fraction of a second before. This is where theory is needed and where our project starts.

For the conditions encountered in the experiments, theoretical predictions of QCD need to be derived non-perturbatively. The best method to date is the numerical evaluation of the Feynman path integral by means of Monte Carlo methods – lattice QCD. Numerically this is a challenging task which requires the highest supercomputing capabilities.

The particular focus of the project described here has been the following: the number of particles leaving the QGP at freeze-out is a couple thousand. While the species composition of the cooled fireball fluctuates from collision to collision, these fluctuations carry precious information. In particular, the fluctuations of the number of heavy, so-called strange and charm quark bound states are interesting. It was the aim of the project to predict these fluctuations as a function of temperature (and chemical potentials) in order to allow, in combination with the experimental data, the determination of the freeze-out temperature for strangeness or charm.

The results of the project revealed that strange as well as charm quarks freeze-out at roughly the same temperature as the light quarks of which protons and neutrons are made. The project, however, also delivered an unexpected side result: when the lattice QCD results are compared to very successful models which describe a hadron gas at high temperatures just below the one where the transition to the QGP takes place, it turns out that these models are missing strange and charm quark bound states, states which have not yet been detected experimentally.

Thus, at high temperature something has been learned also about strong interaction physics at room temperature.

Scientific Contact:

Edwin Laermann 
Fakultät für Physik , Universität Bielefeld 
Universitätsstrasse, D-33615 Bielefeld (Germany)
email: edwin [at] physik.uni-bielefeld.de