Charged Pion Electroproduction Research Articles

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.

Measurement of Transverse Target-spin Asymmetries at Hermes

Azimuthal asymmetries are observed in the cross section of semi-inclusive charged pion electroproduction from a transversely polarized hydrogen target at Hermes. Their si- multaneous dependence on the azimuthal angles of both the pion and the target spin axis allows the extraction of two Fourier components that are interpreted in terms of Collins and Sivers effects. For π + -production both components are found to be positive, while for π − -production the Sivers asymmetry vanishes and the Collins asymmetry is found to be negative and surprisingly large in magnitude.

Single-Spin Asymmetries in Semi-Inclusive Deep-Inelastic Scattering on a Transversely Polarized Hydrogen Target

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. The asymmetry depends on the azimuthal angles of both the pion (phi) and the target spin axis (phi(S)) about the virtual-photon direction and relative to the lepton scattering plane. The extracted Fourier component sin((phi+phi(S))(pi)(UT) is a signal of the previously unmeasured quark transversity distribution, in conjunction with the Collins fragmentation function, also unknown. The component sin((phi-phi(S)(pi)(UT) arises from a correlation between the transverse polarization of the target nucleon and the intrinsic transverse momentum of quarks, as represented by the previously unmeasured Sivers distribution function. Evidence for both signals is observed, but the Sivers asymmetry may be affected by exclusive vector meson production.

New possibilities for studying nucleon resonances in the production of π + π − pairs by polarized electrons on an unpolarized proton

The difference of the cross sections for double charged-pion production in the scattering of opposite-helicity electrons on an unpolarized proton is evaluated within the phenomenological model developed previously. The electromagnetic nucleon-resonance form factors and the parameters of nonresonance processes were taken from a fit to the latest data of the CLAS Collaboration on double charged-pion electroproduction. The effect of the longitudinal excitations of the P11(1440) and D13(1520) states on the difference of the helicity components of the cross section is studied. The sensitivity of this observable to the nucleon-resonance contribution opens the possibility of employing it both in extracting the Coulomb form factors and in seeking new baryon states. Signals from one of these states were possibly observed in the latest data of the CLAS Collaboration.

Revisiting the axial form factor of the nucleon for low Q2 pion electroproduction

An extensive programme of charged pion electroproduction off the proton has been performed at the three-spectrometer facility at the Mainz Microton MAMI. The reaction p( e, e′ π +) n was measured at a constant invariant mass of W = 1125 MeV and four-momentum transfers of Q 2 = 0.058, 0.117, 0.195, and 0.273 (GeV/ c) 2. For each Q 2 value. measurements at different values of ϵ, the virtual photon polarization parameters, allow for a Rosenbluth separation of longitudinal and transverse cross sections. Preliminary results from new measurements in 2000 and 2002 seem to indicate that the transverse cross section at lower Q 2 values is considerably lower than expected from previous experiments.

Spin-independent and double-spin cosφ asymmetries in semi-inclusive pion electroproduction

We consider the cosφ dependence of the longitudinal double-spin asymmetry for charged pion electroproduction in semi-inclusive deep inelastic scattering, emphasizing intrinsic transverse momentum effects. This azimuthal asymmetry allows to measure the cosφ moments of the unpolarized and double-spin cross-section, simultaneously. The size of the asymmetry, in the approximation where all twist-3 interaction-dependent distribution and fragmentation functions are set to zero, is estimated for HERMES kinematics; both the spin-independent and the double-spin cosφ moments are predicted to be sizable and negative.

Measurement of high lying nucleon resonances and search for a missing state in double charged pion electroproduction off the proton

We have analyzed data taken by the CLAS collaboration in double charged pion electroproduction off the proton. We have used an isobar model specifically developed for the analysis of the Nππ channel. This approach relates baryon resonance parameters to the experimental cross section, allowing the extraction of electromagnetic and some strong couplings from the fit to the data. A remarkable feature of the CLAS data is a resonance structure around 1.7 GeV which has not been seen in double pion photoproduction. The structure appears at all Q 2 values from 0.5 to 1.5 GeV 2 and it can not be reproduced with conventional N ∗ parameters.

Longitudinal electroproduction of charged pions from (1)H, (2)H, and (3)He.

Separated longitudinal and transverse cross sections for charged pion electroproduction from (1)H, (2)H, and (3)He were measured at Q(2) = 0.4 (GeV/c)(2) for two values of the invariant mass, W = 1.15 GeV and W = 1.60 GeV, in a search for a mass dependence which would signal the effect of nuclear pions. This is the first such study that includes recoil momenta significantly above the Fermi surface. The longitudinal cross section, if dominated by the pion-pole process, should be sensitive to nuclear pion currents. Comparisons of the longitudinal cross section target ratios to a quasifree calculation reveal a significant suppression in (3)He at W = 1.60 GeV. The W = 1.15 GeV results are consistent with simple estimates of the effect of nuclear pion currents, but are also consistent with pure quasifree production.

Charged pion electroproduction on H, 2H, and 3He

Charged pion electroproduction on H, 2H, and 3He

Pion charge radius from charged pion electroproduction

We analyze a low-energy theorem of threshold pion electroproduction which allows one to determine the charge radius of the pion. We show that at the same order where the radius appears, pion loops induce a correction to the momentum dependence of the longitudinal dipole amplitude $L_{0+}^{(-)}$. This model-independent correction amounts to an increase of the pion charge radius squared from the electroproduction data by about 0.26~fm$^2$. It sheds light on the apparent discrepancy between the recent determination of the pion radius from electroproduction data and the one based on pion-electron scattering.

Regge description of charged pseudoscalar meson electroproduction above the resonance region

A gauge invariant model based on $\ensuremath{\pi}$ and $\ensuremath{\rho}$ Regge trajectory exchanges provides a successful and economical description of the forward charged pion electroproduction reactions above the resonance region. Its sensitivity to the meson form factors is investigated in kinematics where the pion electromagnetic form factor will be extracted at high ${Q}^{2}$ at TJNAF. An extension to ${K}^{+}$ electroproduction is presented.

Coherent electroproduction of pions from 3He.

The $^{3}\mathrm{He}$(e,${\mathit{e}}^{\ensuremath{'}}$${\mathrm{\ensuremath{\pi}}}^{+}$${)}^{3}$H reaction is the simplest of the coherent charged pion electroproduction reactions in nuclear physics. It is sensitive to the structure of the trinucleons, and can be studied with the high duty factor electron accelerators now available. It may also provide some insight for understanding the basic p(e,${\mathit{e}}^{\ensuremath{'}}$${\mathrm{\ensuremath{\pi}}}^{+}$)n reaction if the latter can be described within the constituent quark models of nucleons. In this work, we calculate the ratio of p(e,${\mathit{e}}^{\ensuremath{'}}$${\mathrm{\ensuremath{\pi}}}^{+}$)n and $^{3}\mathrm{He}$(e,${\mathit{e}}^{\ensuremath{'}}$${\mathrm{\ensuremath{\pi}}}^{+}$${)}^{3}$H cross sections for two kinematical conditions with invariant photon-nucleon mass W=1160 and 1230 MeV, respectively. The four important contributions to this ratio are identified as (i) the finite size (i.e., the form factor) of the trinucleons, (ii) the spin-isospin correlations in the trinucleons, (iii) the finite range of the propagation of the struck proton, and (iv) the possible pion-nucleon final state interactions. Together they reduce the ratio of the cross section to \ensuremath{\sim}0.4 in this kinematic range. \textcopyright{} 1996 The American Physical Society.

Dipole strengths in 11Be from electroproduction of charged pions.

The dipole resonances in the light nucleus $^{11}\mathrm{Be}$ were studied measuring the energy spectra and angular distributions of charged pions in the spin-flip charge-exchange reaction $^{11}\mathrm{B}$(e,${\mathrm{\ensuremath{\pi}}}^{+}$${)}^{11}$Be at ${\mathit{E}}_{\mathit{e}}$=200 MeV. The ground state and eight excited states below ${\mathit{E}}_{\mathit{x}}$=18 MeV in $^{11}\mathrm{Be}$ were observed. The angular distributions were analyzed within the framework of the distorted-wave impulse approximation. A considerable amount of the deduced E1 strengths is distributed over excitation energies much lower than 18 MeV in $^{11}\mathrm{Be}$. The energy distributions of the E1 strengths of transitions to the s- and d-shell states were compared with the results of theoretical calculations with the effect of loosely bound neutron states.

Electroproduction of charged pions from light nuclei

Electroproduction of charged pi mesons from light nuclei to discrete final states is studied in a distorted wave impulse approximation. The Blomqvist and Laget pion photoproduction operator is extended to the case of electroproduction by including virtual photons. Gauge invariance is included in the theory by making the Feynman diagrams for the process individually gauge invariant. Conservation of the electroproduction current and isolation of the electron's kinematical aspects are done by introducing modified virtual photon polarization vectors. Cross section calculations for single nucleon electroproduction performed with the real photoproduction parameters of Blomqvist and Laget and standard forms of electromagnetic form factors agree with data. The single nucleon virtual photoproduction operator is used in a distorted wave impulse approximation to describe electroproduction from a nuclear target leading to a discrete final nuclear state. Nuclear transition densities which enter into elastic electron scattering and electroproduction processes are compared. It is found that because of the change of nuclear isospin, electroproduction provides information on a larger number of transition densities than inelastic electron scattering for a given target nucleus. Methods of integrating over the electron's kinematics are presented for single arm experiments, that is, where only the produced pion is detected. Single arm electroproduction cross sections are calculated for 10B and 12C targets. For the 10B case, good agreement with data is found. For the 12C electroproduction cross sections, no conclusions can be reached because the quality of present data is poor. It is found that in these experiments, contributions to the cross section from longitudinal virtual photon components are negligible because of small values of virtual photon invariant mass.

Electroproduction of charged pions from light nuclei: J. L. Sabutis and f. Tabakin. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Electroproduction of charged pions from light nuclei: J. L. Sabutis and f. Tabakin. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Electroproduction of single charged pions from deuterium atQ 2≈1 GeV2 in the resonance region

We present differential cross-sections for the electro-production of single charged pions from deuterium for a virtual photon mass squared −1.0 GeV2 and for pion nucleon masses in the range 1.23–1.68 GeV (the 1st and 2nd resonance regions). The data are compared with predictions from fits to hydrogen data.

Charged-pion electroproduction and spin-isospin modes in nuclei

We study photopionic nuclear processes in the quasielastic region. We show how charged-pion electroproduction allows simultaneous access to the transverse and longitudinal spin-isospin responses. Gauge-invariance requirements are taken into account in a coherent way with the various medium effects.

Electroproduction of charged pions on 12C leading to discrete isobar analogue states

Angular distribution of pions from 12C, leading to the ground state of 12B and 12N and to the first excited state of 12B, have been measured via the (e, π) reaction at T π = 32 MeV. Using the virtual photon theory single differential cross sections are extracted from the measured data. The results are compared with data from other laboratories and with theoretical calculations. The theoretical description of the data is qualitatively very good. Quantitatively the measured data are only partially well described by the theoretical calculations of the different authors.

Electroproduction of single charged pions from deuterons at Q2 ∼0.5 (GeV/ c) 2

The ratio of π − to π + electroproduction cross sections from deuterons has been measured in the resonance region at an average four-momentum transfer squared of 0.5 (GeV/ c) 2. Results are presented over a wide range of pion production angles and comparisons are made with theoretical predictions based on SU(6) w symmetry and the Melosh transformation.

Threshold electroproduction of charged pions and pion condensation precursor phenomena

It is shown that the electroproduction of charged pions near threshold is strongly influenced by pion condensation precursor phenomena and may provide yet another probe of those effects. It is a particularly useful reaction in that it may be subject to relatively reliable theoretical treatment. Sample numerical results are presented here for the particular case of 12C(e, e π +) 12B (1 +, 1) and also for an inclusive measurement in which only the final pion need be observed.