| Research Abstract |
Through a series of evidences, the quark-gluon plasma seems to be created in the early stage of relativistic heavy-ion collisions at RHIC. In order to extract thermodynamic properties of the quark-gluon plasma quantitatively, a lattice calculation of finite temperature QCD-directly from the first principles of QCD-is indispensable. Because the typical energy scale given by the QCD transition temperature is about the strange quark mass, a realistic simulation require a dynamical treatment of the strange quark (three-flavor QCD) besides conventional up and down quarks (two-flavor QCD). Due to various difficulties, however, most large-scale simulations have been done in two-flavor QCD.
Toward a realistic simulation of the quark-gluon plasma, this project aims at a systematic study of three-flavor QCD. We also study efficient methods for thermodynamic quantities in finite-temperature lattice QCD.
As the first step of the three-flavor project, we carried out a systematic comparison of various
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exact algorithms proposed recently for QCD with odd number of flavors. We found a version of the PHMC algorithm is efficient enough for a systematic study of three-flavor QCD. From a preparative simulation using the PHMC algorithm, we also find that improvement of both gauge and quark actions is indispensable for a continuum extrapolation with present power of computers. Finally we determined non-perturbative values of the improvement parameter c_<sw> at relevant coupling parameters.
Based on these results, we started a systematic study of hadronic spectroscopy in three-flavor QCD at zero temperature. We studied at three lattice spacings, 0.122, 0.10, and 0.07 fm, on 16^3x32, 20^3x40, and 24^3x56 lattices, respectively. Simulations on the first two lattices have been completed, and that on the finest lattice on-going. We confirmed that, in contrast to the discrepancy of quenched spectrum observed in a previous study, three-flavor QCD reproduces experimental meson spectrum correctly. We also found that the light u,d quark mass and the s quark mass are about 20--30% smaller than those evaluated previously in the quenched QCD.
We also studied efficient methods to calculate the equation of state and thermodynamic quantities in finite-temperature QCD. In a quenched simulation, we have shown that anisotropic lattices provide us with such an efficient way. Accordingly, we have carried out the first controlled continuum extrapolation of the equation of state using data at three lattice spacings. To carry out a more realistic calculation in QCD with dynamical quarks, we evaluated non-perturvative values of anisotropy coefficients in two-flavor QCD with improved quark and gluon actions. Less
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