Peripheral Meson Model of Deep Inelastic Rapidity Gap Events

We show that a peripheral meson model can explain the large deep inelastic electron-proton scattering rapidity gap events observed at HERA.Comment: 12 pages, 2 figure

On the form factor of physical mesons and their distribution function

This work addresses more to the technical rather than to the physical problem, how to calculate analytically the form factor $F(Q)$, the associated mean-square radius $$, and the distribution function $\Phi(x,Q^2)$ for a given light-cone wave function $\Psi_{q\bar q}(x,\vec k_{\perp})$ of the pion. They turn out to be functions of only one dimensionless parameter, which is the ratio of the constituent quark mass and an effective Bohr momentum which measures the width of the wave function in momentum space. Both parameters are subject to change in the future, when the presently used solution for the over simplified $\uparrow\downarrow$-model will be replaced by something better. Their relation to and agreement with experiment is discussed in detail. The procedure can be generalized also to other hadrons.Comment: 19 pages, 8 figures, 1 table, 22 references. Submitted to Nucl.Phys.

Meson-meson correlations in baryon-baryon and antibaryon-baryon interactions

Recent work of the J\"ulich group about the role of meson-meson correlations in baryon-baryon and antibaryon-baryon interactions is reviewed.Comment: Lecture given at the Erice School 1995, TEX, 10 pages, 15 figure

A possible glueball contribution to the Goldberger-Treiman relations

We discuss the influence of glueball coupling to nucleons on the weak axial-vector coupling constants including singlet channel. We consider a possibility of introduction of constituent gluon contribution to the proton spin. The estimated value for this quantity seems to be rather small.Comment: 7 pages, Latex,(missing table), CAMK 256 199

Coherent η\eta-photoproduction on 4^4He and 12^{12}C in the near-threshold region

Coherent $\eta$ meson photoproduction on $^4$He and $^{12}$C is considered in the near-threshold region. The elementary $\eta$ photoproduction operator includes contributions from the $S_{11}(1535)$ and $D_{13}(1520)$ resonances as well as $t$-channel vector meson exchange and the nucleon pole terms. Due to the suppression of the dominant $S_{11}(1535)$ resonance for spin and isospin saturated nuclei, the reaction is mainly governed by $\omega$ exchange. Furthermore, the influence of Fermi motion and of different prescriptions for the choice of the invariant reaction energy $W_{\gamma N}$ in the elementary amplitude is studied.Comment: 27 pages revtex including 9 postscript figure

Error estimates for pi-pi scattering threshold parameters in Chiral Perturbation Theory to two loops

Using the analysis of ChPT to two loops, we perform an error analysis of the threshold and low energy parameters, based on the uncertainties for the one loop low energy parameters and the resonance saturation mechanism. Different sets of one loop low energy constants have been considered.Thus, the predictive power of the effective field theory is quantified on the basis of the present experimental uncertainties.Comment: 12 pages, 2 tables, 3 figures. Numerics upgraded to v2 of ref.(8). New reference added. Typos corrected. New figure for SU(3) parameters include

Peripheral Nucleon-Nucleon Phase Shifts and Chiral Symmetry

Within the one-loop approximation of baryon chiral perturbation theory we calculate all one-pion and two-pion exchange contributions to the nucleon-nucleon interaction. In fact we construct the elastic NN-scattering amplitude up to and including third order in small momenta. The phase shifts with orbital angular momentum $L\geq2$ and the mixing angles with $J\geq2$ are given parameterfree and thus allow for a detailed test of chiral symmetry in the two-nucleon system. We find that for the D-waves the $2\pi$-exchange corrections are too large as compared with empirical phase shifts, signaling the increasing importance of shorter range effects in lower partial waves. For higher partial waves, especially for G-waves, the model independent $2\pi$-exchange corrections bring the chiral prediction close to empirical NN phase shifts. We propose to use the chiral NN phase shifts with $L\geq 3$ as input in a future phase shift analysis. Furthermore, we compute the irreducible two-pion exchange NN-potentials in coordinate space. They turn out to be of van-der-Waals type, with exponential screening of two-pion mass range.Comment: 30 pages, TeX,12 figures in ps , submitted to Nucl. Phys.

Chiral corrections to kaon-nucleon scattering lengths

We calculate the threshold T-matrices of kaon-nucleon and antikaon-nucleon scattering to one loop order in SU(3) heavy baryon chiral perturbation theory. To that order the complex-valued isospin-1 $\bar KN$ threshold T-matrix can be successfully predicted from the isospin-0 and 1 $KN$ threshold T-matrices. As expected perturbation theory fails to explain the isospin-0 $\bar KN$ threshold T-matrix which is completely dominated by the nearby subthreshold $\Lambda^*(1405)$-resonance. Cancelations of large terms of second and third chiral order are observed as they seem to be typical for SU(3) baryon chiral perturbation theory calculations. We also give the kaon and eta loop corrections to the $\pi N$ scattering lengths and we investigate $\pi\Lambda$ scattering to one-loop order. The second order s-wave low-energy constants are all of natural size and do not exceed 1 GeV$^{-1}$ in magnitude.Comment: 8 pages, 2 figures, published in Phys. Rev. C64, 045204 (2001), corrections of numerical prefactors in Eqs.(10,11,12

pp→ppωpp\to pp\omega reaction near threshold

We analyze the total cross section data for $pp \to pp\omega$ near threshold measured recently at SATURNE. Using an effective range approximation for the on-shell $pp$ S-wave final state interaction we extract from these data the modulus $|\Omega| = 0.53$ fm$^4$ of the threshold transition amplitude $\Omega$. We present a calculation of various (tree-level) meson exchange diagrams contributing to $\Omega$. It is essential that $\omega$-emission from the anomalous $\omega\rho\pi$-vertex interferes destructively with $\omega$-emission from the proton lines. The contribution of scalar $\sigma$-meson exchange to $\Omega$ turns out to be negligibly small. Without introducing off-shell meson-nucleon form factors the experimental value $|\Omega|=0.53$ fm$^4$ can be reproduced with an $\omega N$-coupling constant of $g_{\omega N}=10.7$. The results of the present approach agree qualitatively with the J\"ulich model. We also perform a combined analysis of the reactions $pp\to pp\pi^0, pn\pi^+, pp\eta, pp\omega$ and $pn\to pn\eta$ near threshold.Comment: Latex-file 6 pages, 2 Figure