The hybrid quarkonium states are studied using the Born-Oppenheimer expansion. The first step in this expansion is the determination of the energy levels of the gluons in the presence of a static quark-antiquark pair as a function of the quark-antiquark separation. The spectrum of such gluonic excitations is determined from first principles using lattice QCD.

I analyse the problem of introducing the spin of the quark and the antiquark in effective QCD strings from the point of view of the Wilson loop for a spin-half particle, or the Super Wilson loop. A string representation of the Super Wilson loop is obtained starting from an effective string representation of a Wilson Loop. The action obtained in this manner is invariant under a worldline supersy...

The flow-equation method of continuous unitary transformations is used to eliminate the minimal quark-gluon interaction in the light-front quantized QCD Hamiltonian. The coupled differential equations in the two lowest Fock sectors correspond to the renormalization of the light-front gluon mass and the generation of an effective quark-antiquark (as well as gluon-gluon) interaction. The original...

In this talk, multiquarks are studied microscopically in a standard quark model. In pure ground-state pentaquarks the short-range interaction is computed and it is shown to be repulsive, a narrow pentaquark cannot be in the groundstate. As a possible excitation, an additional quark-antiquark pair is then considered, and this is suggested to produce linear molecular system, with a narrow decay w...

For the first time a rigorous quantum treatment of the LandauPomeranchuk-Migdal effect in QED and QCD is given. The rate of photon (gluon) radiation by an electron (quark) in medium is expressed through the Green’s function of a two-dimensional Schrödinger equation with an imaginary potential. In QED this potential is proportional to the dipole cross section for scattering of ee pair off an ato...

We use the Bohr-Sommerfeld quantization approach in the context of constituent quark models. This method provides, for the Cornell potential, analytical formulas for the energy spectra which closely approximate numerical exact calculations performed with the Schrödinger or the spinless Salpeter equations. The Bohr-Sommerfeld quantization procedure can also be used to calculate other observables...

Within the chiral constituent quark model of Glozman and Riska, we discuss the stability of heavy pentaquarks, i.e. hadrons containing four light quarks and a heavy antiquark. The spin-dependent part of the Hamiltonian is dominated by the short-range part of the Goldstone-boson-exchange interaction. We find that these systems are not bound, having an energy above the lowest dissociation thresho...

Starting from the relativistic gauge-invariant quark-antiquark Green function we obtain the relevant interaction in the one-body limit, which can be interpreted as the kernel of a non-perturbative Dirac equation. We study this kernel in different kinematic regions, reproducing, in particular, for heavy quark the potential case and sum rules results. We discuss the relevance of the result for he...

We study an influence of precise data on uncertainty of polarized parton distribution functions. This analysis includes the SLAC-E155 proton target data which are precise measurements. Polarized PDF uncertainties are estimated by using the Hessian matrix. We examine correlation effect between the antiquark and gluon uncertainties. It suggests that reducing the gluon uncertainty is needed to det...