Thermal dilepton and meson spectral functions from lattice QCD

Pseudo-scalar and vector meson correlation functions were calculated at temperatures below and above the deconfinement transition using O(a) improved Wilson fermions in quenched lattice QCD. The spectral functions were reconstructed from the correlator given only at discrete points in Euclidean time by means of the Maximum Entropy Method without a priori assumptions on the spectral shape. The deviation of the spectral functions from the one of a freely propagating quark anti-quark pair is examined above the critical temperature. At 1.5 and 3 T_c the vector spectral function yields an enhancement of the dilepton rate over the Born rate in the energy interval 4 < #omega#/T < 8 and a sharp drop at energies below 2-3 T. (orig.)Available from TIB Hannover: RA 2999(02-19) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Scaling test for Wilson twisted mass QCD

We present a first scaling test of twisted mass QCD with pure Wilson quarks for a twisting angle of #pi#/2. We have computed the vector meson mass m_V and the pseudoscalar decay constant F_P_S for different values of #beta# at fixed value of r_0m_P_S. The results obtained in the quenched approximation are compared with data for pure Wilson and non-perturbatively O(a) improved Wilson computations. We show that our results from Wilson twisted mass QCD show clearly reduced lattice spacing errors, consistent with O(a) improvement and without the need of any improvement terms added. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(03-198) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The H dibaryon on the lattice

We present our final results for the mass of the six quark flavor singlet state (J"P=0"+, S=-2) called H dibaryon, which would be the lightest possible strangelet in the context of strange quark matter. The calculations are performed in quenched QCD on (8-24)"3 x 30 lattices with the (1,2) Symanzik improved gauge action and the clover fermion action. Furthermore the fuzzing technique for the fermion fields and smearing of the gauge fields is applied in order to enhance the overlap with the ground state. Depending on the lattice size we observe an H mass slightly above or comparable with the #LAMBDA##LAMBDA# threshold for strong decay. Therefore a bound H dibaryon state seemed to be ruled out by our simulation. (orig.)Available from TIB Hannover: RA 2999(02-117) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A study of charmonium systems across the deconfinement transition

We present results from lattice studies of charmonium systems near the deconfinement transition temperature. On quenched isotropic lattices with lattice spacings between 0.02 and 0.05 fm, qq systems with quark masses close to the charm mass and with different spin-parity quantum numbers are studied in the temperature range 0.9T_c-3T_c. Results for temporal correlators of local operators, and the spectral functions constructed from them, are discussed. For the pseudoscalar and vector channels, the correlators are observed to change very little across the deconfinement transition, unlike in the case of the light quarks. (orig.)Available from TIB Hannover: RA 2999(02-108) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Comparing iterative methods for overlap and twisted mass fermions

SIGLEAvailable from TIB Hannover: RA2999(04-185) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Simulating the Schroedinger functional with two pseudo-fermions: Algorithmic study and the running mass

We present an algorithmic study for the simulation of two massless flavors of O(a) improved Wilson quarks with Schroedinger functional boundary conditions. The algorithm used is Hybrid Monte Carlo with two pseudo-fermion fields as proposed by M. Hasenbusch. A gain in CPU cost of a factor two is reached when compared to one pseudo-fermion field due to the larger possible step-size. This study is integrated in the ALPHA project for the computation of the running of the renormalized quark mass. We include an update on these physics results. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(03-137) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Temporal propagators and quasiparticles in hot QCD

We have calculated the temporal quark and gluon propagators on large (64"3 x 16) isotropic lattice and extracted the spectral function from them. Calculating the spectral function at several spatial momenta we were able to determine the dispersion relation for quarks and gluons and extract the temperature dependent quasiparticle masses. We have found that the quasiparticle masses measured in units of temperature increase substantially as the temperature is lowered from 3T_c to 1.5T_c. Our temporal propagators are quite different from those calculated within the HTL perturbation theory. We have found that in general medium effects are stronger than suggested by hard thermal loop perturbation theory. In particular, the quasiparticle masses determined by us are generally larger than those predicted by HTL perturbation theory (except for the gluon mass at 3T_c). We also see a large continuum contribution in the gluon spectral function which is absent in the HTL approximation. (orig.)Available from TIB Hannover: RA 2999(02-109) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A lattice calculation of thermal dilepton rates

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<or#approx##omega#/T<or#approx#8 and suggest that the spectrum is cut-off at low energies by a thermal mass threshold of about (2 - 3) T. (orig.)Available from TIB Hannover: RA 2999(01-175) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Behavior of charmonium systems after deconfinement

We present a study of charmonia in hot gluonic plasma, for temperatures upto 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/#psi# and #eta#_c survive up to quite high temperatures, with little observable change up to 1.5 T_c, and then gradually weakening and disappearing by 3 T_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 1.1 T_c. (orig.)Available from TIB Hannover: RA 2999(03-215) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Hadron correlators, spectral functions and thermal dilepton rates from lattice QCD

We discuss information on thermal modifications of hadron properties which can be extracted from the structure of Euclidean correlation functions of hadronic currents as well as more direct information obtained through the reconstruction of the spectral functions based on the Maximum Entropy Method. (orig.)Available from TIB Hannover: RA 2999(02-153) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman