Effects of Negative Energy Components in the Constituent Quark Model

Relativistic covariance requires that in the constituent quark model for mesons the positive energy states as well as the negative energy states are included. Using relativistic quasi-potential equations the contribution of the negative energy states is studied for the light and charmonium mesons. It is found that these states change the meson mass spectrum significantly but leave its global structure untouched.Comment: 14 pages revtex 3.0, 4 figures uudecoded attached in postscript format, THU-93/1

The electron-nucleon cross section in (e,e′p)(e,e'p) reactions

We examine commonly used approaches to deal with the scattering of electrons from a bound nucleon. Several prescriptions are shown to be related by gauge transformations. Nevertheless, due to current non-conservation, they yield different results. These differences reflect the size of the uncertainty that persists in the interpretation of $(e,e'p)$ experiments.Comment: 6 pp (10 in preprint form), ReVTeX, (+ 4 figures, uuencoded

Magnetic string contribution to hadron dynamics in QCD

Dynamics of a light quark in the field of static source (heavy-light meson) is studied using the nonlinear Dirac equation, derived recently. Special attention is paid to the contribution of magnetic correlators and it is found that it yields a significant increase of string tension at intermediate distances. The spectrum of heavy-light mesons is computed with account of this contribution and compared to experimental and lattice data.Comment: 10 pages Revte

Antiproton-nucleus electromagnetic annihilation as a way to access the proton timelike form factors

Contrary to the reaction pbar + p --> e+ e- with a high momentum incident antiproton on a free target proton at rest, in which the invariant mass M of the (e+ e-) pair is necessarily much larger than the (pbar p) mass, in the reaction pbar + d --> n e+ e- the value of M can take values near or below the (pbar p) mass. In the antiproton-deuteron electromagnetic annihilation, this allows to access the proton electromagnetic form factors in the time-like region of q^2 near the (pbar p) threshold. We estimate the cross section dsigma(pbar +d --> e+ e- n)/dM for an antiproton beam momentum of 1.5 GeV/c. We find that near the (pbar p) threshold this cross section is about 1 pb/MeV. The case of heavy nuclei target is also discussed. Elements of experimental feasibility are presented for the process pbar + d --> n e+ e- in the context of the Panda project.Comment: 14 pages, 11 figures. submitted to EPJ

Role of baryonic resonances in the dilepton emission in nucleon-nucleon collisions

Within an effective Lagrangian model, we present calculations for cross sections of the dilepton production in proton-proton and proton-neutron collisions at laboratory kinetic energies in 1-5 GeV range. Production amplitudes include contributions from the nucleon-nucleon bremsstrahlung as well as from the mechanism of excitation, propagation, and radiative decay of Delta(1232) and N*(1520) intermediate baryonic resonances. It is found that the delta isobar terms dominate the cross sections in the entire considered beam energy range. Our calculations are able to explain the data of the DLS collaboration on the dilepton production in proton-proton collisions for beam energies below 1.3 GeV. However, for incident energies higher than this the inclusion of contributions from other dilepton sources like Dalitz decay of pi0 and eta mesons, and direct decay of rho and omega mesons is necessary to describe the data.Comment: 22 pages, 7 figures, more details of the calculations added, version to appear in Phys. Rev

Relativistic analysis of the constituent quark model for mesons

Extensions of the constituent quark potential model for mesons are formulated and analyzed which include relativistic dynamics and the full Dirac structure of both positive and negative energy states. The meson wave equations are transformed to configuration space where they can easily be solved numerically. Thus we present the first systematic description of the mass spectra of light, mixed and heavy mesons which takes into account the full Dirac structure without approximations. The pion form factor and radiative, leptonic and electroweak meson decay are studied.We find that the projection on the full Dirac structure of the confining potential and of the meson decay vertices gives unexpected large modifications to the nonrelativistic mass spectra and decay vertices