Recent results on screening masses
Recent results for screening masses at high temperature are presented. The spatial correlation functions of operators with meson quantum numbers were obtained on quenched gauge configurations with non-perturbatively improved Wilson quark s as well as, in a different round of analysis, on the NF = 2+ 1 configurations generated in the course of RBC-Bielefeld’s analysis of the Equation of State. While vector and axial vector channels signal the restoration of chiral symmetry in both cases, in 2 + 1 flavor QCD pseudoscalar and isovec tor scalar become degenerate only at a temperature higher than the chiral crossover one, thus indicating that the UA(1) is effectively restored only at higher temperatures. In the inve stigated temperature range pseudoscalar and vector screening masses do not become equal, contrary to the free case and to leading order perturbation theory. For the quenched Wilson case the differences could be established in the infinite volume continuumm limit.

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: Meson screening masses from lattice QCD with two light and the strange quark
abstract: We present results for screening masses of mesons built from light and strange quarks in the temperature range of approximately between 140 MeV to 800 MeV. The lattice computations were performed with 2 + 1 dynamical light and strange flavors of improved (p4) staggered fermions along a line of constant physics defined by a pion mass of about 220 MeV and a kaon mass of 500 MeV. The lattices had temporal extents Nτ = 4, 6 and 8 and aspect ratios of Ns/Nτ ≥ 4. At least up to a temperature of 140 MeV the pseudo-scalar screening mass remains almost equal to the corresponding zero temperature pseudo-scalar (pole) mass. At temperatures around 3Tc (Tc being the transition temperature) the continuum extrapolated pseudo-scalar screening mass approaches very close to the free continuum result of 2πT from below. On the other hand, at high temperatures the vector screening mass turns out to be larger than the free continuum value of 2πT . The pseudo-scalar and the vector screening masses do not become degenerate even for a temperature as high as 4Tc. Using these mesonic spatial correlation functions we have also investigated the restoration of chiral symmetry and the effective restoration of the axial symmetry. We have found that the vector and the axial-vector screening correlators become degenerate, indicating chiral symmetry restoration, at a temperature which is consistent with the QCD transition temperature obtained in previous studies. On the other hand, the pseudo-scalar and the scalar screening correlators become degenerate only at temperatures larger than 1.3Tc, indicating that the effective restoration of the axial symmetry takes place at a temperature larger than the QCD transition temperature. PACS. 11.15.Ha , 11.10.Wx, 12.38.Gc, 12.38.Mh

title: Continuum extrapolation of finite temperature meson correlation functions in quenched lattice QCD
abstract: We explore the continuum limit $a\rightarrow 0$ of meson correlation functions at finite temperature. In detail we analyze finite volume and lattice cut-off effects in view of possible consequences for continuum physics. We perform calculations on quenched gauge configurations using the clover improved Wilson fermion action. We present and discuss simulations on isotropic $N_\sigma^3\times 16$ lattices with $N_\sigma=32,48,64,128$ and $128^3 \times N_\tau$ lattices with $N_\tau=16,24,32,48$ corresponding to lattice spacings in the range of $0.01 fm \lsim a \lsim\ 0.031 fm$ at $T\simeq1.45T_c$. Continuum limit extrapolations of vector meson and pseudo scalar correlators are performed and their large distance expansion in terms of thermal moments is introduced. We discuss consequences of this analysis for the calculation of the electrical conductivity of the QGP at this temperature.

title: Thermal dilepton rate and electrical conductivity: An analysis of vector current correlation functions in quenched lattice QCD
abstract: 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: 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 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.

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.