Thermal dilepton rate and electrical conductivity: An analysis of vector current correlation functions in quenched lattice QCD
We calculate the vector current correlation function for light valence quarks in the deconfined phase of QCD. The calculations have been performed in quenched lattice QCD at T ≃ 1.45Tc for four values of the lattice cut-off on lattices up to size 128 3 × 48. This allows to perform a continuum extrapolation of the correlation function in the Euclidean time interval 0.2 ≤ τT ≤ 0.5, which extends to the largest temporal separations possible at finite temperature, to better than 1% accuracy. In this interval, at the value of the temperature investigated, we find that the vector correlation function never deviates from the free correlator for massless quarks by more than 9%. We also determine the first two non-vanishing thermal moments of the vector meson spectral function. The second thermal moment deviates by less than 7% from the free value. With these constraints, we then proceed to extract information on the spectral representation of the vector correlator and discuss resulting consequences for the electrical conductivity and the thermal dilepton rate in the plasma phase.

title: Heavy quark diffusion from lattice QCD spectral functions
abstract: We analyze the low-frequency part of charmonium spectral functions on large lattices close to the continuum limit in the temperature region 1.5 T/Tc 3 as well as for T 0.75Tc. We present evidence for the existence of a transport peak above Tc and its absence below Tc. The heavy quark diffusion constant is then estimated using the Kubo formula. As part of the calculation we also determine the temperature dependence of the signature for the charmonium bound state in the spectral function and discuss the fate of charmonium states in the hot medium.

title: Towards a non-perturbative measurement of the heavy quark momentum diffusion coefficient
abstract: We report on a lattice investigation of heavy quark diffusion within pure SU(3) plasma above the deconfinement transition, with the quarks treated to leadin g order in the heavy mass expansion. Using a multilevel algorithm, several volumes and lattice spacings, as well as tree-level improvement and perturbative renormalization, we measure the relevant “colour-electric” Euclidean correlator, finding that it clearly exceeds its perturbative co unterpart. Even without analytic continuation, this suggests that at temperatures just above the cri tical one, non-perturbative interactions felt by the heavy quarks are stronger than within the weak-coupling expansion. After introducing rough modelling of the spectral shape, diffusion coefficien ts down to D∼ 0.5/T appear possible.

title: A study of high-energy cosmic-ray interactions at sea level. II. Theory
abstract: For pt.I see ibid., vol.3, p.979 (1977). The expected cosmic-ray burst spectrum ( approximately 1 TeV) in a lead target at sea level has been determined using the published hadronic and muonic spectra. The response of the recording scintillator detector is carefully assessed. The muon burst size spectrum is straightforwardly calculated. Two different approaches are used to determine the expected hadronic burst spectrum, one an analytical technique and the other a Monte-Carlo method. Good agreement between the two techniques is found. The Monte-Carlo method has the advantage of taking into account multiple hadron interactions. The results are compared with experimental data derived from the scintillator detector. The muon-initiated burst spectrum shows close agreement, but a discrepancy of at least a factor of three appears between calculated and measured intensities of hadron-induced bursts.

title: A study of high-energy cosmic-ray interactions at sea level. I. Experimental and interpretation
abstract: The burst-size spectrum due to cosmic rays of energy approximately 1 TeV interacting in a lead target at sea level has been measured. The zenith angle distribution of the particles initiating the bursts has also been determined. The results are compared with predicted spectra based on theoretical calculations. No unusual features are observed and it appears that the observed data could be explained in terms of hadronic and muonic fluxes at sea level. However, the measured hadronic contribution is higher than that predicted by the calculations.