Quark spin and orbital angular momentum in the nucleon are calculated in symmetry-breaking chiral quark model. The results are compared with data and other models. Synopsis: Recent measurements show (1) strong flavor asymmetry of the antiquark sea 1 : ¯ d > ¯ u, (2) nonzero strange quark content 2 : < ¯ ss > = 0, (3) sum of quark spins is small 3 : < s z > q+¯ q = 0.1 − 0.2, and (4) antiquark s...

The spectrum of gluons in the presence of a static quark-antiquark pair is studied using Monte Carlo simulations on anisotropic space-time lattices. For very small quark-antiquark separations R, the level orderings and approximate degeneracies disagree with the expectations from an effective string theory. As the quarkantiquark separation R increases, a dramatic rearrangement of the energies oc...

We consider the possibility that D − D̄ mixing undergoes resonant enhancement. Various scenarios are explored, including the potential effects of nearby pseudoscalar or scalar quark-antiquark (QQ̄) resonances as well as of hybrid quark-antiquark-gluon QQ̄G states. Assuming reasonable values for the resonance parameters we find relatively sizeable contributions from individual resonances to both ∆m...

We investigate soft gluon radiation off a quark–antiquark antenna in both color singlet and octet configurations traversing a dense medium. We demonstrate that, in both cases, multiple scatterings lead to a gradual decoherence of the antenna radiation as a function of the medium density. In particular, in the limit of an completely opaque medium, total decoherence is obtained, i.e., the quark a...

AdS/QCD correspondence allows to compute the quark-antiquark potential in the static limit. We use this piece of information together with the Salpeter equation (Schroedinger equation with relativistic kinematics) and a short range hyperfine splitting potential to describe the bound states of quark-antiquark. Comparison with experiment permits to fit the parameters of the model, namely the quar...

Within an effective field theory framework we study heavy-quark–antiquark systems with a typical distance between the heavy quark and the antiquark smaller than 1/ΛQCD. A suitable definition of the potential is given within this framework, while non-potential (retardation) effects are taken into account in a systematic way. We explore different physical systems. Model-independent results on the...

The recent data on the polarization of vector meson at LEP show that the vector mesons favor the helicity zero state. We calculate the helicity density matrix of vector meson which contains a polarized fragmenting quark by adding the spin of the fragmenting quark and that of the antiquark created in the fragmentation. The data at LEP imply a significant polarization for the antiquark in the opp...

The gluon excitations of the QCD vacuum are investigated in the presence of a static quark-antiquark source. It is shown that the ground state potential and the excitation spectrum of dynamical gluon degrees of freedom, as determined in our lattice simulations, agree remarkably well with model predictions based on the diaelectric properties of the confining vacuum described as a dual supercondu...

We study exotic static 3-body potentials, utilizing generalized Wilson Loops in SU(3) lattice QCD. For the quark-antiquark-gluon techniques we address the angles of 0, 45, 60, 90, 120, 135 and 180 degrees, between the quark-gluon and the antiquark-gluon segments. We calculate the form of the static potential and discuss whether, or not, two-body interactions exist between the three different bo...

Inclusive and diffractive structure functions for electron-proton scattering are calculated in a semiclassical approach at large momentum transfer Q2 and small values of the scaling variable x. The basic process is the production of a quark-antiquark pair in the colour field of the proton. The structure functions are expressed in terms of Wilson lines along the classical trajectories of quark a...