Lattice QCD Thermodynamics on the Grid

TL;DR

This paper details the use of large-scale Grid computing resources to perform extensive lattice QCD simulations, revealing insights into the QCD phase transition and its implications for experimental searches.

Contribution

It demonstrates efficient utilization of Grid resources for large-scale QCD simulations and combines supercomputing with distributed Grid computing for complex physics calculations.

Findings

QCD undergoes a second-order phase transition at high temperature.

The transition appears smoothened with increased quark density.

Results suggest the QCD critical point may be unlikely at small chemical potential.

Abstract

We describe how we have used simultaneously ${\cal O}(10^3)$ nodes of the EGEE Grid, accumulating ca. 300 CPU-years in 2-3 months, to determine an important property of Quantum Chromodynamics. We explain how Grid resources were exploited efficiently and with ease, using user-level overlay based on Ganga and DIANE tools above standard Grid software stack. Application-specific scheduling and resource selection based on simple but powerful heuristics allowed to improve efficiency of the processing to obtain desired scientific results by a specified deadline. This is also a demonstration of combined use of supercomputers, to calculate the initial state of the QCD system, and Grids, to perform the subsequent massively distributed simulations. The QCD simulation was performed on a $16^3\times 4$ lattice. Keeping the strange quark mass at its physical value, we reduced the masses of the up and down quarks until, under an increase of temperature, the system underwent a second-order phase transition to a quark-gluon plasma. Then we measured the response of this system to an increase in the quark density. We find that the transition is smoothened rather than sharpened. If confirmed on a finer lattice, this finding makes it unlikely for ongoing experimental searches to find a QCD critical point at small chemical potential.