A Consistent Meson-Field-Theoretical Description of PP-Bremsstrahlung

A parameter-free and relativistic extension of the RuhrPot meson-baryon model is used to define the dominant isoscalar meson-exchange currents. We compute pp-bremsstrahlung observables below the $\pi-$production threshold using a relativistic hadronic current density that includes impulse, wave function re-orthonormalization, meson-recoil, $\bar{{\rm N}}$N creation and annihilation, $\rho\pi\gamma$ + $\omega\pi\gamma$ + $\rho\eta\gamma$ + $\omega\eta\gamma$ vector-meson decay and N$\Delta\gamma(\pi,\rho)$ exchange currents. We obtain a good description of the available data. The N$\Delta\gamma(\pi)$ current is shown to dominate the large two-body contributions and closed-form expressions for various non-relativistic approximations are analyzed. An experimental sensitivity to the admixture of pseudo-scalar and pseudo-vector admixture of the NN$\pi$ interaction is demonstrated. We examine the Lorentz invariance of the NN$\rightleftharpoons$NN $t$-matrices and show a dominantly pseudo-vector NN$\pi$ coupling renders impulse approximation calculations without boost operators to be essentially exact. Conversely, a similar analysis of the $\Delta{\rm N}\rightleftharpoons$NN transitions shows that boost operators and the two-body $N\Delta\gamma$ wave function re-orthonormalization meson-recoil currents are required in NN, $\Delta$N and $\Delta\Delta$ coupled channel $t$-matrix applications. The need for additional data is stressed.Comment: 34 pages of uuencoded REVTeX using multicol.sty with 21 figures inlined using epsf.sty. This revision adds section IV.

High-Q2Q^2 Elastic eded-scattering and QCD predictions

In the framework of pertubative QCD it is argued that in the elastic $ed$-scattering at $Q^{2}\sim$ few $(GeV/c)^{2}$ the light-cone-frame helicity-flip amplitudes could not be omitted. The obtained $\frac{B}{A}$ ratio of Rosenbluth structure functions is shown to be in a good agreement with experimental data. The high $Q^2$ behavior of tensor analysing power $T_{20}$ is discussed.Comment: 6 pages + 2 ps figures not included, LaTeX, ITP-93-33

Soft pion theorem for hard near threshold pion production

We prove new soft pion theorem for the near threshold pion production by a hard electromagnetic probe. This theorem relates various near threshold pion production amplitudes to the nucleon distribution amplitudes. The new soft pion theorem is in a good agreement with the SLAC data for F_2^p(W,Q^2) for W^2 < 1.4 GeV^2 and 7 < Q^2 < 30.7 GeV^2.Comment: 9 pages, revised version, more general analysi

Scaling law for the electromagnetic form factors of the proton

The violation of the scaling law for the electric and magnetic form factors of the proton are examined within the cloudy bag model. The suppression of the ratio of the electric and magnetic form factors is natural in the bag model. The pion cloud plays a moderate role in understanding the recent data from TJNAF.Comment: 8 pages, REVTeX, 2 figures include

Octet-Baryon Form Factors in the Diquark Model

We present an alternative parameterization of the quark-diquark model of baryons which particularly takes care of the most recent proton electric form-factor data from the E136 experiment at SLAC. In addition to electromagnetic form factors of the nucleon, for which good agreement with data is achieved, we discuss the weak axial vector form factor of the nucleon as well as electromagnetic form factors of $\Lambda$ and $\Sigma$ hyperons. Technical advance in calculating the pertinent analytic expressions within perturbative quantum chromodynamics is gained by formulating the wave function of the quark-diquark system in a covariant way. Finally, we also comment on the influence of Sudakov corrections within the scope of the diquark model.Comment: 16 pages, WU-B 93-07, latex, uuencoded postscript files of 7 figures appended at the end of the latex fil

Recalculation of Proton Compton Scattering in Perturbative QCD

At very high energy and wide angles, Compton scattering on the proton (gamma p -> gamma p) is described by perturbative QCD. The perturbative QCD calculation has been performed several times previously, at leading twist and at leading order in alpha_s, with mutually inconsistent results, even when the same light-cone distribution amplitudes have been employed. We have recalculated the helicity amplitudes for this process, using contour deformations to evaluate the singular integrals over the light-cone momentum fractions. We do not obtain complete agreement with any previous result. Our results are closest to those of the most recent previous computation, differing significantly for just one of the three independent helicity amplitudes, and only for backward scattering angles. We present results for the unpolarized cross section, and for three different polarization asymmetries. We compare the perturbative QCD predictions for these observables with those of the handbag and diquark models. In order to reduce uncertainties associated with alpha_s and the three-quark wave function normalization, we have normalized the Compton cross section using the proton elastic form factor. The theoretical predictions for this ratio are about an order of magnitude below existing experimental data.Comment: Latex, 23 pages, 13 figures. Checked numerical integration one more way; added results for one more proton distribution amplitude; a few other minor changes. Version to appear in Phys. Rev.