Research Objectives
Our objectives are to determine the position and nature of the phase transition from hadronic matter to a quark-gluon plasma from lattice quantum chromodynamics (QCD), to determine properties of hadrons from lattice QCD, and to improve lattice QCD methodology.
Computational Approach
QCD is simulated on a discrete space-time lattice (lattice QCD) by formulating it as a molecular dynamics problem and solving its "time" evolution numerically. A stochastic driving term is applied periodically to ensure ergodicity, and rapid progress through phase space. The Dirac operator which describes quark propagation is inverted numerically using a conjugate gradient method. These computations have been ported to the T3E using the natural parallelism associated with the lattice, and the locality of ite interactions.
Accomplishments
We have probed the chiral nature of the QCD phase transition at finite temperature by studying the screening lengths for mesonic exitations in the hadronic matter and plasma phases. This gave evidence that the flavour singlet axial current is not restored at this transition.
By adding a four-quark interaction to our lattice action, we have been able to study the lattice QCD phase transition for zero quark mass. This will give us direct access to the scaling properties at this transition, and ultimately the equation of state for hadronic matter and the quark-gluon plasma.
We developed a new class of lattice staggered quark actions which have less flavour symmetry breaking due to discretization errors.
Significance
The finite temperature phase transition is expected to be observed in relativistic heavy ion collider experiments. It is also important for the understanding of the early universe.
Improved actions allow the use of coarser lattices, thus reducing the computing requirements for simulations.
Publications
J. B. Kogut, J.-F. Lagae and D. K.i Sinclair. 1997. Topology, fermionic zero modes and flavor singlet correlators in finite temperature QCD. E-print hep-lat/9709067 and ANL-HEP-CP-97-4. http://www-spires.slac.stanford.edu/spires/form/hepspif.html
J.-F. Lagae and D. K. Sinclair. 1997. Improving the staggered quark action to reduce flavour symmetry violations. E-print hep-lat/9709035. http://www-spires.slac.stanford.edu/spires/form/hepspif.html
J. B. Kogut and D. K. Sinclair. 1997. QCD with chiral four fermion interactions. Nuclear Physics (Proc. Suppl.) 53:272.
Order parameters near the finite temperature phase transition in lattice
QCD with explicit 4-fermion couplings.