P. Stoler

Measurements of Deuteron Photodisintegration up to 4.0 GeV

The first measurements of the differential cross section for the d(γ,p)n reaction up to 4.0 GeV were performed at the Continuous Electron Beam Accelerator Facility (CEBAF) at Thomas Jefferson Laboratory. We report the cross sections at the proton center-of-mass angles of 36°, 52°, 69°, and 89°. These results are in reasonable agreement with previous measurements at lower energy. The 89° and 69° data show constituent-counting-rule behavior up to 4.0 GeV photon energy. The 52° and 36° data disagree with the counting-rule behavior. The quantum chromodynamics (QCD) model of nuclear reactions involving reduced amplitudes disagrees with the present data.

Electroproduction of the Delta (1232) resonance at high momentum transfer

The differential cross section for the process p(e, e{prime}p){eta} has been measured at Q{sup 2}=2.4 and 3.6 (GeV/c){sup 2} at center-of-mass energies encompassing the S{sub 11}(1535) resonance. The latter point is the highest-Q{sup 2} exclusive measurement of this process to date. The resonance width and the helicity-1/2 transition amplitude are extracted from the data, and evidence for the possible onset of scaling in this reaction is shown. A lower bound of approximately 0.45 is placed on the S{sub 11}(1535) {r_arrow} p{eta} branching fraction.

Baryon form factors at high Q2 and the transition to perturbative QCD

Abstract The status of electromagnetic form-factors of nucleons and non-strange nucleon resonances is rewiewed. Of special interest is the question of what regions in Q2 correspond to the breakdown of the validity of constituent quark models, and the region of Q2 corresponding to the onset of the validity of perturbative QCD. Theoretical models based on perturbative QCD are reviewed and results of calculations compared to available experimental data. It is observed that relativistic constituent quark models become invalid for Q2 greater than about 2 GeV2/c2, compared with available experimental data. The Q2 dependence of experimental form-factors appears to begin to follow perturbative QCD scaling rules for Q2 as low as 5 or 6 GeV2/c2. A notable exception is the δ(1232), whose form-f actor decreases faster than other states. However the normalized magnitudes of form-factors at experimentally accessible values of Q2 obtained with perturbative QCD sum rule techniques is controversial and currently unsettled. The diquark model is discussed as a possible bridge between low and high-Q2 kinematic regions.

HADRONIC FORM FACTORS AND PERTURBATIVE QCD

▪ Abstract The electromagnetic form factors of hadrons at large momentum transfer have been the subject of intense theoretical and experimental scrutiny over the past two decades, yet there is still not a universally accepted framework for their description. This review is a synopsis of their current status. The basic theoretical approaches to form factors at large momentum transfer are developed, emphasizing the valence quark and Feynman (soft) pictures. The discussion includes the relation of these descriptions to the parton model, as well as the roles of factorization, evolution, Sudakov resummation and QCD sum rules. This is followed by a discussion of the experimental status of pion and nucleon elastic form factors and resonance production amplitudes in the light of recent data; the successes and shortcomings of various theoretical proposals are highlighted.

Transverse momentum dependence of semi-inclusive pion production

Abstract Cross sections for semi-inclusive electroproduction of charged pions ( π ± ) from both proton and deuteron targets were measured for 0.2 x 0.5 , 2 Q 2 4 GeV 2 , 0.3 z 1 , and P t 2 0.2 GeV 2 . For P t 0.1 GeV , we find the azimuthal dependence to be small, as expected theoretically. For both π + and π − , the P t dependence from the deuteron is found to be slightly weaker than from the proton. In the context of a simple model, this implies that the initial transverse momenta width of d quarks is larger than for u quarks and, contrary to expectations, the transverse momentum width of the favored fragmentation function is larger than the unfavored one.

The CLAS Cherenkov detector

The design, construction, and performance of the CLAS Cherenkov threshold gas detector at Jefferson Lab is described. The detector consists of 216 optical modules. Each module consists of three adjustable mirrors of lightweight composite construction, a Winston light collecting cone, a 5-in. photomultiplier tube, and specially designed magnetic shielding. Efficiencies and response functions have been measured. # 2001 Elsevier Science B.V. All rights reserved. PACS: 95.55.Vj

Compton-scattering cross section on the proton at high momentum transfer.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5-11 and -t=2-7 GeV2 with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0+/-0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

Semi-Inclusive Charged-Pion Electroproduction off Protons and Deuterons: Cross Sections, Ratios and Access to the Quark-Parton Model at Low Energies

A large set of cross sections for semi-inclusive electroproduction of charged pions (π^±) from both proton and deuteron targets was measured. The data are in the deep-inelastic scattering region with invariant mass squared W^2>4 GeV^2 (up to ≈7 GeV^2) and range in four-momentum transfer squared 2<Q^2<4 (GeV/c)^2, and cover a range in the Bjorken scaling variable 0.2<x<0.6. The fractional energy of the pions spans a range 0.3<z<1, with small transverse momenta with respect to the virtual-photon direction, Pt^(2)_(t)<0.2 (GeV/c)2. The invariant mass that goes undetected, M_x or W′, is in the nucleon resonance region, W′<2 GeV. The new data conclusively show the onset of quark-hadron duality in this process, and the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms. The x, z, and Pt^(2)_(t) dependences of several ratios (the ratios of favored-unfavored fragmentation functions, charged pion ratios, deuteron-hydrogen and aluminum-deuteron ratios for π^+ and π^−) have been studied. The ratios are found to be in good agreement with expectations based upon a high-energy quark-parton model description. We find the azimuthal dependences to be small, as compared to exclusive pion electroproduction, and consistent with theoretical expectations based on tree-level factorization in terms of transverse-momentum-dependent parton distribution and fragmentation functions. In the context of a simple model, the initial transverse momenta of d quarks are found to be slightly smaller than for u quarks, while the transverse momentum width of the favored fragmentation function is about the same as for the unfavored one, and both fragmentation widths are larger than the quark widths.

Baryon form-factors at high momentum transfer and generalized parton distributions

Nucleon form factors at high momentum transfer |t| are treated in the framework of generalized parton distributions (GPDs). The possibility of obtaining information about parton high transverse momentum components by application of GPDs to form factors is discussed. This is illustrated by applying an ad-hoc 2-body parton wave function to elastic nucleon form factors F1 and F2, the N to Delta magnetic form factor G*M, and wide angle Compton scattering (WACS) form factor R1.

First measurement of the reaction 3He(γ,pp)n

Abstract Differential cross sections for the 3 He(γ, pp)n reaction were measured with a bremsstrahlung beam for kinematics selected in the quasi-free two-nucleon region with neutron recoil momentum p n ≈ 0. The data confirm the quenching of the cross section, with respect to the (γ, pn) reaction, predicted by a diagrammatic description of the process which approximately reproduces the shape of the proton momentum distribution.