Measurements of inclusive pion electroproduction

Measurements of the inclusive pion electroproduction reaction e^?+p → e^?+π^±+X made at the Wilson Synchrotron Laboratory are reported. Data were taken at points with nominal (Q², W) values of (0.6, 2.67), (1.2, 2.67), (2.0, 2.67) and (1.2, 2.15) in (GeV², GeV). A more complete angular distribution was taken at (1.2, 2.67). The virtual photoproduction cross section for π⁺ + missing mass is studied as a function of missing mass, Feynman scaling variables, Q² and W. The invariant cross section show a simple behavior as a function of Q² and W. Its dependenc on P_⊥², x′ and φ is determined.     

Inelastic response functions for pion electroproduction near threshold

Longitudinal and transverse response functions are calculated for pion electroproduction from nuclei near threshold, for an experimental situation in which only the final electron is detected. For the case of calcium, a Fermi gas model is used, and the calculated response functions and cross sections are at least an order of magnitude smaller than the measured quasielastic contributions, indicating that electroproduction is a relatively unimportant background. For the case of oxygen, it is found that pion electroproduction accompanied by excitation of collective GDR states differs in magnitude and spectrum shape from that predicted by a quasifree model.     

Exclusive electroproduction of pion pairs

We investigate electroproduction of pion pairs on the nucleon in the framework of QCD factorization for hard exclusive processes. We extend previous analyses by taking the hard-scattering coefficients at next-to-leading order in α_s . The dynamics of the produced pion pair is described by two-pion distribution amplitudes, for which we perform a detailed theoretical and phenomenological analysis. In particular, we obtain constraints on these quantities by comparing our results with measurements of angular observables that are sensitive to the interference between two-pion production in the isoscalar and isovector channels.     

Hard exclusive electroproduction of pions

We investigate the exclusive electroproduction of mesons from nucleons. To leading-twist, leading-order accuracy, the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms of quark double distribution functions of the nucleon. While the pseudovector contribution is connected to ordinary polarized quark distributions, the pseudoscalar part can be related to one-pion t-channel exchange. We observe that the pseudovector part of the production amplitude is important at . On the other hand, for , contributions from one-pion exchange dominate. Received: 4 January 1999 / Published online: 15 July 1999     

A dispersive approach to pion photo- and electroproduction

The relativistic amplitudes of pion photo- and electroproduction are calculated by dispersion relations at constant t. Several sum rules and low-energy theorems for the threshold amplitudes are investigated within this technique. The continuation of the amplitudes to sub-threshold kinematics is shown to provide a unique framework to derive the low-energy constants of chiral perturbation theory by global properties of the excitation spectrum.     

Onset of quark-hadron duality in pion electroproduction

A large data set of charged-pion (pi+/-) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark-->pion production mechanisms.     

Determination of neutral current couplings from neutrino-induced semi-inclusive pion and inclusive reactions

We show that by looking at data from neutrino-induced semi-inclusive pion and inclusive reactions on isoscalar targets alone, one can determine completely the neutral current couplings. Predictions for various models are also presented.     

Effects of the inclusive dipole-pomeron to exclusive processes

The consequences of the inclusive dipole-Pomeron for the exclusive processes are investigated. In the weak coupling limit a two component picture for the production mechanism is obtained and self-consistency of the inclusive Pomeron dipole is found.     

Semi-inclusive neutrino scattering and pion structure functions

We show that a study of the processes

can provide information on the pion structure functions. Using a realistic parton distribution for the pion we calculate cross sections and relevant distributions for these processes. The results show that such measurements are experimentally accessible.     

Deeply virtual and exclusive electroproduction of ω-mesons

The exclusive ω electroproduction off the proton was studied in a large kinematical domain above the nucleon resonance region and for the highest possible photon virtuality (Q²) with the 5.75 GeV beam at CEBAF and the CLAS spectrometer. Cross-sections were measured up to large values of the four-momentum transfer (- t < 2.7 GeV²) to the proton. The contributions of the interference terms σ_{TT} and σ_{TL} to the cross-sections, as well as an analysis of the ω spin density matrix, indicate that helicity is not conserved in this process. The t-channel π⁰ exchange, or more generally the exchange of the associated Regge trajectory, seems to dominate the reaction γ^*p↦ωp, even for Q² as large as 5 GeV². Contributions of handbag diagrams, related to Generalized Parton Distributions in the nucleon, are therefore difficult to extract for this process. Remarkably, the high-t behaviour of the cross-sections is nearly Q²-independent, which may be interpreted as a coupling of the photon to a point-like object in this kinematical limit.     

Transversity in hard exclusive electroproduction of pseudoscalar mesons

Estimates for electroproduction of pseudoscalar mesons at small values of skewness are presented. Cross-sections and asymmetries for these processes are calculated within the handbag approach which is based on factorization in hard parton subprocesses and soft generalized parton distributions (GPDs). The latter are constructed from double distributions. Transversity GPDs are taken into account; they are accompanied by twist-3 meson wave functions. For most pseudoscalar-meson channels a combination of ˜_T and [`(E)]_T \bar{{E}}_{T}^{} plays a particularly prominent role. This combination of GPDs, which we constrain by moments obtained from lattice QCD, leads, with the exception of the p⁺ \pi^{+}_{} and h^￠ \eta^{\prime}_{} channels, to large transverse cross-sections.     

Implications of gauge invariance for pion electroproduction at threshold

We discuss the implications of gauge invariance in the problem of the on-shell extrapolation of the electroproduction low-energy theorems. We show that there is an invariant amplitude which can be evaluated at the Breit threshold either using gauge invariance and on-shell dispersion relations or following the Fubini and Furlan [5] extrapolation method starting from the current-algebra low-energy value of the amplitude. Comparing the two expressions, we find a relation between the electromagnetic pion form factor, F_π (k²), and the axial-vector nucleon form factors, g_A (k²) and h_A (k²).     

Analysis of pion electroproduction data in terms of a pion distribution function of the proton

Experimental data on the longitudinal π⁺ electroproduction on hydrogen have been analyzed in terms of a pion distribution function of the proton. This interpretation is based on the assumption that the pion can be considered as a parton in the nucleon for low momentum transfer Q². For the kinematical conditions photon mass Q² < 1 GeV² and electron energy loss ν > 2.2 GeV the pion electroproduction in the direction of the virtual photon can be viewed upon as quasielastic eπ⁺ → e′π⁺ scattering. Using the impulse approximation the pion distribution function of the proton can be deduced. According to the distribution function evaluated from pion electroproduction in the infinite momentum frame, the physical proton has a 3% admixture of the nπ⁺ state and the π⁺ in the nπ⁺ state carries 0.6% of the proton momentum.     

Manifestations of the weak interaction in pion electroproduction on nucleons

A study is made of the simplest (pole) mechanisms of violation of the invariance under spatial reflection near the threshold of pion electroproduction on nucleons. A detailed investigation is made of the Born mechanism of violation of P invariance and also vector meson exchange with violation of P invariance at the electromagnetic or the strong vertices. The contribution of isobars is calculated in the framework of the isobar model in the resonance region of energies.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 3–10, April, 1980.     

t/s scaling in exclusive and inclusive proton reactions

A model is developed in order to account for the previously observed large-angle behaviour in elastic scattering, and to predict the analogous inclusive effects. Data of (pp, p) distributions and pp → pN^* reactions are examined in regions of high transverse momenta and fitted by the functional forms of the model. The fits show the expected features of this new type of high-energy scaling.     

Pion electromagnetic structure functions from pion exchange in leptoproduction

The pion-exchange contribution to e+p→e′+n+X is calculated using different models, both for pion exchange and for the pion electromagnetic structure functions. Numerical estimates of this contribution to deep inelastic scattering off protons are given.