Target mass effects in polarized electroproduction

We examine the target mass effects in spin-dependent deep inelastic electroproduction within the framework of Wilson's operator product expansion. Simple analytic expressions for the Nachtmann moments of polarized structure functions g₁ and g₂ are obtained and their Q² evolutions are considered in QCD. The Bjorken and Burkhardt-Cottingham sum rules including target mass effects are derived and the QCD $\text{g}{}^2$ corrections are discussed.     

On the phenomenological approach to dual resonance models for deep inelastic electroproduction

We outline a systematic procedure for constructing a dual resonance model for deep inelastic electroproduction which incorporates locality, current algebra, Regge behaviour, Bjorken scaling and the Dashen — Fubini — Gell-Mann sum rule. We illustrate the procedure in terms of a simple Ansatz for theW ₂ electroproduction structure function.     

A model of deep inelastic polarized electroproduction

A realistic phenomenological model combining parton/QCD ideas with lower energy SU(6) constraints is proposed for the shape and evolution of the leading spin-dependent structure function G^ep(x, Q²) in polarized electroproduction. Close's broken-SU(6) ansatz is used to relate appropriately defined polarized quark-parton distribution densities to unpolarized ones at the matching momentum scale Q² = Q₀². The differences between spin and helicity distribution densities as well as the complications due to perturbative QCD and parton k_T (with related target-mass) effects are taken into account. Evolution to higher (>10 GeV²) values of Q² (where target-mass effects can be neglected) yields experimentally testable numerical predictions that are presented through various plots. The value of Q₀ is self-consistently determined to be about 0.5 GeV.     

Transversity distribution and polarized fragmentation function from semi-inclusive pion electroproduction

A method is discussed to determine the hitherto unknown u-quark transversity distribution from a planned HERMES measurement of a single target-spin asymmetry in semi-inclusive pion electroproduction off a transversely polarized target. Assuming u-quark dominance, the measurement yields the shapes of the transversity distribution and of the ratio , of polarized and unpolarized u-quark fragmentation functions. The unknown relative normalization can be obtained by identifying the transversity distribution with the well-known helicity distribution at large x and small . The systematic uncertainty of the method is dominated by the assumption of u-quark dominance. Received: 18 June 2000 / Revised version: 20 November 2000 / Published online: 5 February 2001     

Chiral structure of the Roper resonance using complex-mass scheme

The pole mass and the width of the Roper resonance are calculated as functions of the pion mass in the framework of low-energy effective field theory of the strong interactions. We implement a systematic power-counting procedure by applying the complex-mass scheme.     

The swelling of nucleons in nuclei and the Roper resonance

Conditions where some swelling of the nucleon occurs, and, in particular the relation of this effect with the attractive character of the force acting on it, are studied. It is found that short range repulsive correlations can turn the swelling into a shrinking, in spite of a globally attractive interaction, whereas repulsive velocity dependent forces can lead to some swelling. The role of the Roper resonance in this nucleon change of size is considered in some detail.     

The Roper resonance in a finite volume

We calculate the energy levels corresponding to the Roper resonance based on a two-flavor chiral effective Lagrangian for pions, nucleons, deltas, and the Roper resonance at the leading one-loop order. We show that the Roper mass can be extracted from these levels for lattice volumes of moderate size.     

Roper resonances in chiral quark models

We derive a method to calculate the multi-channel K -matrix applicable to a broad class of models in which mesons linearly couple to the quark core. The method is used to calculate pion scattering amplitudes in the energy region of low-lying P11 and P33 resonances. A good agreement with experimental data is achieved if in addition to the elastic channel we include the and N ( channels where the -meson models the correlated two-pion decay. We solve the integral equation for the K -matrix in the approximation of separable kernels; it yields a sizable increase of the widths of the (1232) and the N(1440) resonances compared to the bare quark values.     

Proton sea quark flavour asymmetry and Roper resonance

We study the proton and the Roper resonance together with the meson cloud model, by constructing a Hamiltonian matrix and solving the eigenvalue equation. The proton sea quark flavour asymmetry and some properties of the Roper resonance are thus reproduced in one scheme.     

Coincidence π electroproduction in the second resonance region

The reaction e⁻ + p → e⁻ + p + π^o has been studied in the region of the second pion nucleon resonance. Two sets of data have been taken and analysed at nominal values of the squared four momentum transfer to the pion nucleon system of 0.4 and 0.6 (GeV/c)². A phenomenological interpretation of the data is presented. Our results are compared with the predictions of fixed-t dispersion relation calculations and with the symmetric quark model.     

Determination of the longitudinal and transverse cross section in resonance electroproduction

Using all relevant data we determine σ_L/σ_T for 1.1<W<1.8 GeV, 0.1<Q ²<2.0 (GeV/c)². New results, emerging in the region of the second and third resonance, are mainly due to our recently published very low ε-data. σ_L/σ_T is found to be small but different from zero.     

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.