A Gauge and Lorentz covariant Approximation for the Quark Propagator in an arbitrary Gluon Field

We decompose the quark propagator in the presence of an arbitrary gluon field with respect to a set of Dirac matrices. The four-dimensional integrals which arise in first order perturbation theory are rewritten as line-integrals along certain field lines, together with a weighted integration over the various field lines. It is then easy to transform the propagator into a form involving path ordered exponentials. The resulting expression is non-perturbative and has the correct behavior under Lorentz transformations, gauge transformations and charge conjugation. Furthermore it coincides with the exact propagator in first order of the coupling g. No expansion with respect to the inverse quark mass is involved, the expression can even be used for vanishing mass. For large mass the field lines concentrate near the straight line connection and simple results can be obtained immediately.Comment: 15 pages, 3 figures. Replaced with version to appear in Eur. Phys. J.

On the Glue Content in Heavy Quarkonia

Starting with two coupled Bethe-Salpeter equations for the quark-antiquark, and for the quark-glue-antiquark component of the quarkonium, we solve the bound state equations perturbatively. The resulting admixture of glue can be partially understood in a semiclassical way, one has, however, to take care of the different use of time ordered versus retarded Green functions. Subtle questions concerning the precise definition of the equal time wave function arise, because the wave function for the Coulomb gluon is discontinuous with respect to the relative time of the gluon. A striking feature is that a one loop non abelian graph contributes to the same order as tree graphs, because the couplings of transverse gluons in the tree graphs are suppressed in the non relativistic bound state, while the higher order loop graph can couple to quarks via non suppressed Coulomb gluons. We also calculate the amplitude for quark and antiquark at zero distance in the quark-glue-antiquark component of the P-state. This quantity is of importance for annihilation decays of P-states. It shows a remarkable compensation between the tree graph and the non abelian loop graph contribution. An extension of our results to include non perturbative effects is possible.Comment: 15 pages, 8 figure

Spectroscopy of B_c Mesons in the Relativized Quark Model

We calculate the spectrum of the charm-beauty mesons using the relativized quark model. Using the wavefunctions from this model we compute the radiative widths of excited c\bar{b} states. The hadronic transition rates between c\bar{b} states are estimated using the Kuang-Yan approach and are combined with the radiative widths to give estimates of the relative branching ratios. These results are combined with production rates at the Tevatron and the LHC to suggest promising signals for excited B_c states. Our results are compared with other models to gauge the reliability of the predictions and point out differences.Comment: 15 pages, 1 fig. uses revtex4. References adde

The 1P quarkonium fine splittings at NLO

We calculate the 1P heavy quarkonium fine splittings at NLO and discuss the impact of the calculation on the chi_b(1P) splittings.Comment: 10 pages, 3 figure

Quark-antiquark pair production in space-time dependent fields

Fermion-antifermion pair-production in the presence of classical fields is described based on the retarded and advanced fermion propagators. They are obtained by solving the equation of motion for the Dirac Green's functions with the respective boundary conditions to all orders in the field. Subsequently, various approximation schemes fit for different field configurations are explained. This includes longitudinally boost-invariant forms. Those occur frequently in the description of ultrarelativistic heavy-ion collisions in the semiclassical limit. As a next step, the gauge invariance of the expression for the expectation value of the number of produced fermion-antifermion pairs as a functional of said propagators is investigated in detail. Finally, the calculations are carried out for a longitudinally boost-invariant model-field, taking care of the last issue, especially.Comment: 32 pages, 8 figures, revised versio

Heat transfer and Fourier's law in off-equilibrium systems

We study the most suitable procedure to measure the effective temperature in off-equilibrium systems. We analyze the stationary current established between an off-equilibrium system and a thermometer and the necessary conditions for that current to vanish. We find that the thermometer must have a short characteristic time-scale compared to the typical decorrelation time of the glassy system to correctly measure the effective temperature. This general conclusion is confirmed analyzing an ensemble of harmonic oscillators with Monte Carlo dynamics as an illustrative example of a solvable model of a glass. We also find that the current defined allows to extend Fourier's law to the off-equilibrium regime by consistently defining effective transport coefficients. Our results for the oscillator model explain why thermal conductivities between thermalized and frozen degrees of freedom in structural glasses are extremely small.Comment: 7 pages, REVTeX, 4 eps figure

Nonperturbative QCD Vacuum Effects in Nonlocal Quark Dynamics

A straightforward calculation reveals the essentially nonlocal character of the leading heavy $Q\bar{Q}$ interaction arising from nonperturbative gluon field correlations in the model of a fluctuating QCD vacuum. In light of this quarkonium spin splitting ratio predictions which have supported the scalar confinement ansatz are reconsidered as a specific example of possible consequences for spectroscopy.Comment: Latex, 9 page

Pentaquarks uuddqˉuudd\bar q with One Color Sextet Diquark

The masses of pentaquarks $uudd\bar s$ are calculated within the framework of a semirelativistic effective QCD Hamiltonian, using a diquark picture. This approximation allows a correct treatment of the confinement, assumed here to be similar to a Y-junction. With only color antitriplet diquarks, the mass of the pentaquark candidate $\Theta$ with positive parity is found around 2.2 GeV. It is shown that, if a color sextet diquark is present, the lowest $uudd\bar s$ pentaquark is characterized by a much smaller mass with a negative parity. A mass below 1.7 GeV is computed, if the masses of the color antitriplet and color sextet diquarks are taken similar

Meson-like Baryons and the Spin-Orbit Puzzle

I describe a special class of meson-like \Lambda_Q excited states and present evidence supporting the similarity of their spin-independent spectra to those of mesons. I then examine spin-dependent forces in these baryons, showing that predicted effects of spin-orbit forces are small for them for the same reason they are small for the analogous mesons: a fortuitous cancellation between large spin-orbit forces due to one-gluon-exchange and equally large inverted spin-orbit forces due to Thomas precession in the confining potential. In addition to eliminating the baryon spin-orbit puzzle in these states, this solution provides a new perspective on spin-orbit forces in all baryons.Comment: 24 pages, 4 figure