We present a study of charmonia in hot gluonic plasma, for temperatures up to three times the deconfinement transition temperature T c . qq systems with quark masses close to the charm mass and different spin-parity quantum numbers were studied on very fine isotropic lattices. The analysis of temporal correlators, and spectral functions constructed from them, shows that the J/ and c survive up to quite high temperatures, with little observable change up to 1.5T c , and then gradually weaken and disappear by 3T c . For the scalar and axial vector channels, serious modifications are induced by the hot medium already close to T c , possibly dissociating the mesons by 1.1T c .
Using clover improved Wilson fermions we calculate thermal vector meson correlation functions above the deconfinement phase transition of quenched QCD. At temperatures 1.5 T c and 3 T c they are found to differ by less than 15% from that of a freely propagating quark anti-quark pair. This puts severe constraints on the dilepton production rate and in particular rules out a strong divergence of the dilepton rate at low energies. The vector spectral function, which has been reconstructed using the Maximum Entropy Method, yields an enhancement of the dilepton rate over the Born rate of at most a factor two in the energy interval 4 < ∼ ω/T < ∼ 8 and suggests that the spectrum is cut-off at low energies by a thermal mass threshold of about (2 − 3) T .
The phase structure of zero temperature twisted mass lattice QCD is investigated. We find strong metastabilities in the plaquette observable when the untwisted quark mass assumes positive or negative values. We provide interpretations of this phenomenon in terms of chiral symmetry breaking and the effective potential model of Sharpe and Singleton.
The running of renormalized quark masses is computed in lattice QCD with two
flavors of massless O(a) improved Wilson quarks. The regularization and flavor
independent factor that relates running quark masses to the renormalization
group invariant ones is evaluated in the Schroedinger Functional scheme. Using
existing data for the scale r_0 and the pseudoscalar meson masses, we define a
reference quark mass in QCD with two degenerate quark flavors. We then compute
the renormalization group invariant reference quark mass at three different
lattice spacings. Our estimate for the continuum value is converted to the
strange quark mass with the help of chiral perturbation theory.Comment: 25 pages, 6 figures; sections 1 and 4 rearranged, minor change to the
summary plo
The effect of changing the lattice action for the gluon field on the recently observed [1] first order phase transition near zero quark mass is investigated by replacing the Wilson plaquette action by the DBW2 action. The lattice action for quarks is unchanged: it is in both cases the original Wilson action. It turns out that Wilson fermions with the DBW2 gauge action have a phase structure where the minimal pion mass and the jump of the average plaquette are decreased, when compared to Wilson fermions with Wilson plaquette action at similar values of the lattice spacing. Taking the DBW2 gauge action is advantageous also from the point of view of the computational costs of numerical simulations.
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