skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: gamma* N --> Delta at JLab: Exploring the High Q2 Regime

Abstract

We report a new measurement of the exclusive electroproduction reaction gamma* p --> pi0 p to explore the evolution from soft non-perturbative physics to hard processes via the Q2 dependence of the magnetic (M1+), electric (E1+) and scalar (S1+) multipoles in the N --> Delta transition. 9000 differential cross section data points cover W from threshold to 1.4 GeV/c2, 4pi center-of-mass solid angle, and Q2 from 3 to 6 GeV2/c2, the highest yet achieved. It is found that the magnetic form factor G M * decreases with Q2 more steeply than the proton magnetic form factor, the ratio E1+/M1+ is small and negative, indicating strong helicity non-conservation, and the ratio S1+/M1+ is negative, while its magnitude increases with Q2.

Authors:
; ;
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility, Newport News, VA
Sponsoring Org.:
USDOE - Office of Energy Research (ER)
OSTI Identifier:
910569
Report Number(s):
JLAB-PHY-07-684; DOE/ER/40150-4296
Journal ID: ISSN 0094-243X; APCPCS; TRN: US0704259
DOE Contract Number:
AC05-84ER40150
Resource Type:
Conference
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 904; Conference: Workshop on the Shape of Hadrons, 27-29 Apr 2006, Athens, Greece
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; DIFFERENTIAL CROSS SECTIONS; ELECTROPRODUCTION; FORM FACTORS; HADRONS; HELICITY; MULTIPOLES; PHYSICS; PROTONS; SCALARS; SHAPE

Citation Formats

Maurizio Ungaro, Kyungseon Jo, and Paul Stoler. gamma* N --> Delta at JLab: Exploring the High Q2 Regime. United States: N. p., 2007. Web. doi:10.1063/1.2734308.
Maurizio Ungaro, Kyungseon Jo, & Paul Stoler. gamma* N --> Delta at JLab: Exploring the High Q2 Regime. United States. doi:10.1063/1.2734308.
Maurizio Ungaro, Kyungseon Jo, and Paul Stoler. Sun . "gamma* N --> Delta at JLab: Exploring the High Q2 Regime". United States. doi:10.1063/1.2734308. https://www.osti.gov/servlets/purl/910569.
@article{osti_910569,
title = {gamma* N --> Delta at JLab: Exploring the High Q2 Regime},
author = {Maurizio Ungaro and Kyungseon Jo and Paul Stoler},
abstractNote = {We report a new measurement of the exclusive electroproduction reaction gamma* p --> pi0 p to explore the evolution from soft non-perturbative physics to hard processes via the Q2 dependence of the magnetic (M1+), electric (E1+) and scalar (S1+) multipoles in the N --> Delta transition. 9000 differential cross section data points cover W from threshold to 1.4 GeV/c2, 4pi center-of-mass solid angle, and Q2 from 3 to 6 GeV2/c2, the highest yet achieved. It is found that the magnetic form factor GM* decreases with Q2 more steeply than the proton magnetic form factor, the ratio E1+/M1+ is small and negative, indicating strong helicity non-conservation, and the ratio S1+/M1+ is negative, while its magnitude increases with Q2.},
doi = {10.1063/1.2734308},
journal = {AIP Conference Proceedings},
number = ,
volume = 904,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2007},
month = {Sun Apr 01 00:00:00 EDT 2007}
}

Conference:
Other availability
Please see Document Availability for additional information on obtaining the full-text document. Library patrons may search WorldCat to identify libraries that hold this conference proceeding.

Save / Share:
  • Precision measurements of the p(e, e[prime] p)pi0 reaction were performed using the CEBAF Large Acceptance Spectrometer (CLAS) in the range of photon invariant momentum transfer Q2 = 0.16 – 0.36 GeV2. Fits to the data using both a Unitary Isobar Model and the Sato-Lee dynamical model are used to estimate the `dressed' and `bare' electromagnetic couplings for the magnetic and quadrupole gamma* p --> Delta(1232) transition form factors.
  • A fundamental issue in hadron physics is which degrees of freedom are appropriate to describe exclusive reactions at experimentally accessible momentum transfers, and how are models at low, medium and high Q{sup 2} related? Constituent quark models (CQM) appear to work well at the low Q{sup 2} limit, and it is widely believed that valance pQCD will be valid in the limit of high Q{sup 2}. How far the validity of these models extend in Q{sup 2} is still an open question. Many exclusive reactions exhibit constituent scaling behavior at moderate Q{sup 2} ( few GeV{sup 2}/c{sup 2}) which ismore » interpreted by some [1] as the onset of perturbative QCD. Others [2] argue that the observed scaling is not a manifestation of pQCD, and that at the experimentally accessible range of Q{sup 2} exclusive reactions are explained primarily by soft Feynman mechanism. Recently[3][4], there have been promising developments in bridging the high and low Q{sup 2} extremes for exclusive reactions in terms of a quark-parton description of exclusive reactions In this approach the perturbative hard part of the reaction, which is calculable, is isolated from the non-perturbative soft, physics which is parameterized in terms of non-forward parton distributions (NFPD). These NFPD's are generalizations of the usual parton non-spin and spin distribution functions obtained in unpolarized and polarized deep inelastic inclusive scattering, and in fact reduce to them in the limit of forward scattering. An attractive aspect of this approach is that the same NFPD can be carried over to a variety of exclusive reactions which involve the same set of hadrons, and can be constrained in a number of different experiments which are discussed in these proceedings.« less
  • In this talk, results were presented from two recently published Jefferson Lab experiments where longitudinally polarized electrons were scattered from a polarized {sup 3}He target in the inclusive reaction {sup 3}{rvec H}e({rvec e}, e{prime}) in the deep-inelastic region. Incident electrons with energies E = 3.5-5.7 GeV were scattered from polarized {sup 3}He nuclei whose spins could be oriented parallel or perpendicular to the incident electron momentum, in the scattering plane of the electron. The helicities of the incident electrons were flipped pseudo-randomly at a rate of 30 Hz to minimize helicity correlated systematic uncertainties. A feedback system was also usedmore » to keep the helicity-dependent beam charge asymmetry below 50 x 10{sup -6} for a typical run in the g{sub 2} measurement. Scattered electrons were detected in either one of two nearly identical spectrometers. The detector package included vertical drift chambers for particle tracking, two segmented scintillator trigger planes, and used a gas Cherenkov detector and lead-glass calorimeter for particle identification. Electron polarization was measured periodically using a Moeller polarimeter and monitored continuously using a Compton polarimeter. To study polarized neutrons, a target containing {sup 3}He nuclei was polarized using spin-exchange optical pumping. The {sup 3}He ground state is dominated by the S-state in which the two proton spins are anti-aligned and the spin of the nucleus is carried entirely by the neutron. To obtain neutron information, a correction based on the {sup 3}He wavefunction was applied to the measured {sup 3}He data. The {sup 3}He was contained in a sealed, two-chambered, aluminosilicate glass cell, along with a small quantity of N{sub 2} and Rb to aid in the polarization process. Polarized {sup 3}He is produced in the spherical upper chamber by first polarizing Rb atoms with optical pumping. These atoms can transfer their spin to the {sup 3}He nucleus during binary collisions. Incident electrons scatter from the polarized {sup 3}He in the cylindrical lower chamber which is 40 cm long with side walls of thickness {approx} 1.0 mm and end windows of thickness {approx} 130 {micro}m. The {sup 3}He density as seen by the beam is 2.9 x 10{sup 20}/cm{sup 3}. The average in-beam target polarization was P{sub t} = (40.0 {+-} 1.4)% as measured using both nuclear magnetic resonance and electron paramagnetic resonance.« less
  • In the spring of 2006, Jefferson Lab experiment 02-â 013 successfully collected data to measure the neutron elastic form factor G E n at the four-momentum transfer values Q2 = 1.2, 1.8, 2.6, and 3.5(GeV/c)2. This measurement of the quasi-elastic semi-exclusive 3H-vector e(e-vector ,e[prime]n) reaction used the polarized CEBAF beam (Pb [approximate] 85%) and a highly polarized 3He target (Pt [approximate] 50%). Neutrons were detected in a large array of scintillators with an efficiency of 35â -40%. The electrons were detected by the newly commissioned BigBite spectrometer with a solid angle of 76msr and a momentum resolution of 1%. Frommore » the transverse asymmetry of the cross section, G E n will be extracted. A statistical accuracy of DeltaG E n/G E n [approximate] 0.14 is expected.« less
  • A covariant spectator quark model is applied to study the {gamma}N {yields} N*(1535) reaction in the large Q{sup 2} region. Starting from the relation between the nucleon and N*(1535) systems, the N*(1535) valence quark wave function is determined without the addition of any parameters. The model is then used to calculate the {gamma}N {yields} N*(1535) transition form factors. A very interesting, useful relation between the A{sub 1/2} and S{sub 1/2} helicity amplitudes for Q{sup 2} > GeV{sup 2}, is also derived.