A gauge-invariant chiral unitary framework for kaon photo- and electroproduction on the proton

We present a gauge-invariant approach to photoproduction of mesons on nucleons within a chiral unitary framework. The interaction kernel for meson-baryon scattering is derived from the chiral effective Lagrangian and iterated in a Bethe-Salpeter equation. Within the leading-order approximation to the interaction kernel, data on kaon photoproduction from SAPHIR, CLAS and CBELSA/TAPS are analyzed in the threshold region. The importance of gauge invariance and the precision of various approximations in the interaction kernel utilized in earlier works are discussed.     

Complex mixing of baryon resonances

We employ the Wigner-Weisskopf method for a simultaneous treatment of decays, mass splittings and mixings of baryon resonances. Complex mixing matrices naturally arise in this context. Most of the amplitudes at resonance become also complex now and we make predictions for their phases. In spite of the doubling of the number of parameters in the mixing matrices the well0known discrepancies between decay and spectrum analyses still persist.     

Three-flavor quark mass dependence of baryon spectra in holographic QCD

We introduce the strange quark mass to the Sakai–Sugimoto model of holographic QCD. We compute mass shifts in the spectra of three-flavor baryons at the leading order in perturbation in quark masses. Comparison with experimental data shows an agreement only qualitatively.     

A simple quark model for resonance electroproduction

A quark model for resonance electroproduction is developed which is based on the successful photoproduction model of Kubata and Ohta. A detailed comparison is made between the predictions of the model and the results of recent phenomenological analyses of electroproduction data.     

Bounds on non-leading QCD effects in a quark model for inclusive reactions

We study the possible effect of QCD in the proton wave function in a quark model for inclusive processesA+B→C+X pursued by us. The assumption is the validity of the conjecture of Lepage and Brodsky in QCD on such effects. Our results obey the perturbative expectation, |R _g tan² φ|<1. Symmetric version of the model is, however, found to be at variance with most of the inclusive data as well as with some known phenomenology. If the dynamics of the underlying theory generate Regge-like symmetry breaking approximately, the model is phenomenologically viable, and the non-leading QCD effects become reasonable in size. Phenomenological necessity of the admixtures of (56, 0⁺)*, (70, 0⁺) and (56, 2⁺) in the nucleon wave function is also discussed in the present analysis.     

The masses of the nonstrange baryon resonances in quark model with spin-spin and tensor interactions

We calculate the masses of the nonstrange baryon resonances from a quark model with Coulomb and linear potential and hyperfine interaction. We treat the nonrelativistic problem with help of the variational method, the matrix of the spin-spin and tensor force is diagonalized within a harmonic oscillator supermultiplet. We find for the even and odd parity resonances good agreement with the spectrum of the particle data group and the results of the Karlsruhe-Helsinki phase shift analysis.     

On the choice of quark currents in the QCD sum rules for baryon masses

The cryteria for the choice of the most suitable quark currents in the QCD sum rules for baryon masses are discussed. The currents used in [1, 2] are preferable comparing with those adopted in [3, 4] since the latters result in large power corrections.     

A relativized quark model for radiative baryon transitions

In this paper we investigate the electromagnetic form factors of baryons and their resonances using the framework of a relativized constituent quark model. Beyond the usual single-quark transition ansatz, we incorporate relativistic corrections which are welldetermined by the intrinsic interaction and confinement forces between the quarks. Furthermore we separate off for the compound three-quark system the relativistic center-of-mass motion by an approximately Lorentz-invariant approach. In this way for the first time recoil effects could be explicity studied. Using the harmonic oscillator wavefunctions with the configuration mixing as derived in the Isgur-Karl model, after restoring gauge invariance our relativized interaction hamiltonian can be used to calculate the transversely and longitudinally polarized photon transition form factors of the baryons.     

Medium effects in coherent pion photo- and electroproduction on He and C

Coherent π⁰ photo- and electroproduction on ⁴He and ¹²C nuclei is investigated in the framework of a distorted wave impulse approximation in momentum space. The elementary process is described by the recently developed unitary isobar model. Medium effects are considered by introducing a phenomenological Δ self-energy. Recent experimental data for ⁴He and ¹²C can be well described over a wide range of energies and emission angles by the assumption that the Δ–nuclear interaction saturates.     

QCD at high baryon density in a random matrix model

A high-density diquark phase seems to be a generic feature of QCD. If so it should also be reproduced by random matrix models. We discuss a specific one in which the random matrix elements of the Dirac operator are supplemented by a finite chemical potential and by non-random elements which model the formation of instanton-anti-instanton molecules. Comparing our results to those found in a previous investigation by Vanderheyden and Jackson we find additional support for our starting assumption, namely that the existence of a high-density diquark phase is common to all QCD-like model. Received: 20 February 2001 / Accepted: 24 April 2001     

Photoproduction of baryon resonances in the Skyrme model

We compare the photoproduction helicity amplitudes calculated from the Skyrme model with experimental curves for L_π ⩽ 3 up to k_γ ⩽ 1.25 GeV. In most cases the model gives a good account of structure, size and phases of the measured data.     

Evaluation of multipoles for photo- and electroproduction of pions

With this paper we start a systematic study of the multipole amplitudes for electro- and photoproduction of pions in the low energy region. It is based on the integral equations following from fixedt dispersion relations and the Watson theorem. We start with a discussion of general mathematical problems one encounters when solving the singular integral equations. Especially the non-uniqueness of their solutions poses difficult problems which cannot be solved within the framework of the dispersion theory. As boundary conditions “from outside” we use at threshold the predictions of the static theory. But thereby only the magnetic dipole excitation of the first isobar is determined. For the electric quadrupole amplitude one needs additional conditions.     

Evaluation of multipoles for photo- and electroproduction of pions

In this paper we present the detailed results of an evaluation of the multipole amplitudes for photoproduction of pions on nucleons. We have solved the dispersion relations for the multipolesE ₀₊,M _1?,M ₁₊,E ₁₊,E _2?2 andM _2? by the Muskhelishvili-Omnès method using an appropriate ansatz for the inelastic multipole amplitudes. Four phenomenological parameters occuring in our theory have been determined by a least squares fit of theπ ⁰ experimental different cross section data. Except at pion CM-angles between 50°0 and 80°0 and at 180°0 inπ ⁰ production there is satisfactory agreement between theory and experiment for all measured differential cross sections, polarizations and asymmetries ofπ ⁰- andπ ⁺-photoproduction.     

Nonperturbative quark dynamics in a baryon

The field-correlator method is used to calculate nonperturbative dynamics of quarks in a baryon. The general expression for the 3q Green’s function is obtained using the Fock-Feynman-Schwinger (world-line) path-integral formalism, where all dynamics is contained in the 3q Wilson loop with spin-field insertions. Using the lowest cumulant contribution for the Wilson loop, one obtains a Y-shaped string potential vanishing at the string-junction position. Using the einbein formalism for the quark kinetic terms, one automatically obtains constituent quark masses, calculable through the string tension. The resulting effective action for 3q plus Y-shaped strings is quantized in the path-integral formalism to produce two versions of Hamiltonian: one is in the c.m. and the other is in the light-cone system. The hyperspherical formalism is used to calculate masses and wave functions. Simple estimates in the lowest approximation yield baryon masses in good agreement with experiment without fitting parameters.