Contributions of exclusive pion electroproduction reactions to inclusive structure functions

The contributions of exclusive π⁺n, π⁰p, and π^?π⁺p channels to the inclusive structure function F ₂(x, Q ²) are determined from the CLAS collaboration data analysis. Reliability of the CLAS data on the inclusive structure function and single-pion electroproduction channels is verified. The total contribution of the uninvestigated π⁺π⁰n and π⁰p channels to this function and the integrated cross sections of these channels at photon virtualities Q ² = 0.4 and 0.5 GeV² are predicted.     

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.     

Virtual photons in pion electroproduction

We examine the applicability of the virtual photon spectrum to pion production experiments by using an exact calculation of pion electroproduction from the proton and from ³He. After deriving a new virtual spectrum formula which takes into account the recoil of light target nuclei and the proper electron kinematics near the endpoint, we find that the concept of a virtual photon spectrum in such experiments is very good.     

Invariant amplitudes for pion electroproduction

The invariant amplitudes for pion electroproduction on the nucleon are evaluated by dispersion relations at constant t with MAID as input for the imaginary parts of these amplitudes. In the threshold region these amplitudes are confronted with the predictions of several low-energy theorems derived in the soft-pion limit. In general agreement with chiral perturbation theory, the dispersive approach yields large corrections to these theorems because of the finite pion mass.     

Scaling in deep inelastic pion electroproduction

An attempt is made to apply a light cone dominance technique to the deep inelastic electroproduction of pions. A scaling law is derived which is in agreement with preliminary data.     

Regge cuts and neutral pion electroproduction

A complete FESR-plus-Regge analysis is made of neutral pion photoproduction and electroproduction to understand the Regge cut and its dependence onk ², the square of the virtual photon mass. A SCRAM type of model is essential, but contrary to previous expectations the range of the photon-proton interaction which gives rise to the cut increases with-k ², and is not related to the pointlike, scaling behaviour of deep inelastic scattering, which arises from much simpler parton-model diagrams.     

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.     

Parity violating pion electroproduction off the nucleon

Parity violating (PV) contributions due to interference betweenγ and Z⁰ exchange are calculated for pion electroproduction off the nucleon. A phenomenological model with effective Lagrangians is used to determine the resulting asymmetry for the energy region between threshold andΔ(1232) resonance. TheΔ resonance is treated as a Rarita-Schwinger field with phenomenological NΔ transition currents. The background contributions are given by the usual Born terms using the pseudovectorπN Lagrangian. Numerical results for the asymmetry are presented.     

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.     

Recoil polarization for delta excitation in pion electroproduction

We measured angular distributions of recoil-polarization response functions for neutral pion electroproduction for W = 1.23 GeV at Q(2) = 1.0 (GeV/c)(2), obtaining 14 separated response functions plus 2 Rosenbluth combinations; of these, 12 have been observed for the first time. Dynamical models do not describe quantities governed by imaginary parts of interference products well, indicating the need for adjusting magnitudes and phases for nonresonant amplitudes. We performed a nearly model-independent multipole analysis and obtained values for Re (S(1+)/M(1+)) = -(6.84 +/- 0.15)% and Re (E(1+)/M(1+)) = -(2.91 +/- 0.19)% that are distinctly different from those from the traditional Legendre analysis based upon M1+ dominance and ll(pi) < or = 1 truncation.     

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.     

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.     

Chiral symmetry and diffractive neutral pion photo- and electroproduction

We show that diffractive production of a single neutral pion in photon-induced reactions at high energy is dynamically suppressed due to the approximate chiral symmetry of QCD. These reactions have been proposed as a test of the odderon-exchange mechanism. We show that the odderon contribution to the amplitude for such reactions vanishes exactly in the chiral limit. This result is obtained in a nonperturbative framework and by using PCAC relations between the amplitudes for neutral pion and axial vector current production.     

Contact electromagnetic terms in inclusive photo- and electroproduction

It is shown that contact electromagnetic terms in the photo- and electroproduction exclusive channels can account for a substantial amount of the inclusive real and virtual γp total cross sections. In particular, one gets σ(γp)_exp?σVMD (?, ω, φ) + σ(contact) for Q²=0 and hence no infinite vector mesons (GVMD) seem to be required. The same result is valid for Q²<0. A parallelism with the parton approach is noted.     

Charged pion electroproduction on light nuclei near the threshold

The experimental data for charged pion electroproduction on ¹²C and ¹⁶O measured at Mainz and Sendai are analyzed in a modified equivalent photon approximation. For the Sendai experiment we use distorted waves for the pion, while for the Mainz experiment we adopt a closure sum with a finite Fermi-gas model. We can roughly reproduce both sets of experimental data.     

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.     

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