Hot and dense nuclear matter in particle physics and cosmology
|Scientific Discipline||Particle Physics, Nuclear Physics|
|Principal Investigator(s)||Kari Rummukainen|
|Leading Institution||University of Oulu, Finland|
|DEISA Home Site||CSC|
Project summary and results
Hot and dense nuclear matter in particle physics and Cosmology Quantum Chromodynamics (QCD) is the theory of strong nuclear interactions. At high temperatures and densities the behaviour of matter is determined by QCD; these conditions can be found in the early Universe and in supernova explosions and neutron stars. These conditions are also studied experimentally in so-called Heavy Ion collision experiments. At high enough temperature or density the matter will undergo a phase transition into a novel quark-gluon plasma phase. We are studying the properties of the quark-gluon plasma with large scale lattice Monte Carlo simulations using effective field theories, which succesfully combine analytical and numerical methods. These effective theories are valid at high temperatures and densities, and they are orders of magnitude more economical to simulate than the original full QCD, allowing for precise results at the relevant physical domain. One of the main applications for this method is to study how physical quantities, for example, the pressure, deviate from the predictions given by the analytical perturbation theory. This allows us to obtain a precise equation of state for hot QCD, relevant for precision calculations in Cosmology. Other applications include the study of the plasma instabilities in Heavy Ion collisions.
In the DEISA project we have concentrated on the determination of the quark number susceptibility in hot QCD, both at vanishing and small but non-zero baryon number densities. Susceptibility is a fundamental property of the quark-gluon plasma, and it may affect the baryon number fluctuations in Heavy Ion collision experiments. The lattice simulations have been now done using the resources offered by DEISA and by CSC, Scientific Compting Ltd., Finland. Preliminary analysis has been done for the vanishing baryon number density case; for non-zero baryon number density the analysis is under progress. Some of the preliminary results have already been published in, and the final publication will be ready in summer 2007. For the vanishing baryon number density the simulations indicate that the susceptibility is slightly below the perturbative result. These results are numerically much more accurate than the standard 4-dimensional lattice QCD simulation results.
Role of DEISA facilities
The simulations used for calculating the quark number susceptibility have been performed using IBM p690 -class computers at Finnish center for scientific computing, CSC, and using DEISA resources at CINECA. The total CPU-time used is 125 000 processor-hours, using 32 or 64 processors at a time. The simulation program is a fully parallel lattice code, using asynchronous MPI for internode communications. More or less the same program code has been used in several parallel architectures, and it has excellent scaling properties. For the POWER architecture, it has assembly-language routines for the central routines (3 x 3 complex matrix multiplies).
The DEISA computing facilities have been very beneficial in offering a large cpu-time quota which has been continuously available, without long queuing times. This makes it possible to do large simulation projects in a single concerted push. This is very suitable for well-defined projects which can reliably estimate the required cpu-time. This was the case for our quark number susceptibility project.