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Title: Exclusive single pion electroproduction off the proton: Results from CLAS

Abstract

Exclusive meson electroproduction off protons is a powerful tool to probe the effective degrees of freedom in excited nucleon states at the varying distance scale where the transition from the contributions of both quark core and meson-baryon cloud to the quark core dominance. During the past decade, the CLAS collaboration has executed a broad experimental program to study the excited states of the proton using polarized electron beam and both polarized and unpolarized proton targets. The measurements covered a broad kinematic range in the invariant mass $W$ and photon virtuality $Q^2$ with nearly full coverage in polar and azimuthal angles in the hadronic CM system. As results, several low-lying nucleon resonance states in particular from pion threshold to $W < 1.6$ GeV have been explored. These include $$\Delta$$(1232)$$\frac{3}{2}^+$$, $$N(1440)\frac{1}{2}^+$$, $$N(1520)\frac{3}{2}^-$$, and $$N(1535)\frac{1}{2}^-$$ states. In addition, we recently published the differential cross sections and helicity amplitudes of the reaction $$\gamma^*p\to n\pi^+$$ at higher $W$ (1.6 to 2.0 GeV) which are the $$N(1675)\frac{5}{2}^-$$, $$N(1680)\frac{5}{2}^+$$, and $$N(1710)\frac{1}{2}^+$$ states. These excited states with isospin $1/2$ and with masses near 1.7 GeV can be accessed in single $$n\pi^+$$ production as there are no isospin $3/2$ states present in this mass range with the same spin-parity assignments. As a result, I will briefly discuss these states from CLAS results of the single charged pion electroproduction data.

Authors:
ORCiD logo [1]
  1. Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility, Newport News, VA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)
OSTI Identifier:
1310442
Report Number(s):
JLAB-PHY-16-2249; DOE/OR/23177-3854
Journal ID: ISSN 0177-7963; PII: 1145
Grant/Contract Number:
AC05-06OR23177
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Few-Body Systems
Additional Journal Information:
Conference: ECT* 2015 Workshop on Nucleon Resonances, Trento (Italy), 19-23 Oct 2015; Journal ID: ISSN 0177-7963
Publisher:
Springer
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; CLAS; single pion electroproduction; helicity amplitude; N (1440)1/2+; N (1520)3/2-; N (1535)1/2-; N (1675)5/2-; N (1680)5/2+; N (1710)1/2+

Citation Formats

Park, Kijun. Exclusive single pion electroproduction off the proton: Results from CLAS. United States: N. p., 2016. Web. doi:10.1007/s00601-016-1145-6.
Park, Kijun. Exclusive single pion electroproduction off the proton: Results from CLAS. United States. doi:10.1007/s00601-016-1145-6.
Park, Kijun. 2016. "Exclusive single pion electroproduction off the proton: Results from CLAS". United States. doi:10.1007/s00601-016-1145-6. https://www.osti.gov/servlets/purl/1310442.
@article{osti_1310442,
title = {Exclusive single pion electroproduction off the proton: Results from CLAS},
author = {Park, Kijun},
abstractNote = {Exclusive meson electroproduction off protons is a powerful tool to probe the effective degrees of freedom in excited nucleon states at the varying distance scale where the transition from the contributions of both quark core and meson-baryon cloud to the quark core dominance. During the past decade, the CLAS collaboration has executed a broad experimental program to study the excited states of the proton using polarized electron beam and both polarized and unpolarized proton targets. The measurements covered a broad kinematic range in the invariant mass $W$ and photon virtuality $Q^2$ with nearly full coverage in polar and azimuthal angles in the hadronic CM system. As results, several low-lying nucleon resonance states in particular from pion threshold to $W < 1.6$ GeV have been explored. These include $\Delta$(1232)$\frac{3}{2}^+$, $N(1440)\frac{1}{2}^+$, $N(1520)\frac{3}{2}^-$, and $N(1535)\frac{1}{2}^-$ states. In addition, we recently published the differential cross sections and helicity amplitudes of the reaction $\gamma^*p\to n\pi^+$ at higher $W$ (1.6 to 2.0 GeV) which are the $N(1675)\frac{5}{2}^-$, $N(1680)\frac{5}{2}^+$, and $N(1710)\frac{1}{2}^+$ states. These excited states with isospin $1/2$ and with masses near 1.7 GeV can be accessed in single $n\pi^+$ production as there are no isospin $3/2$ states present in this mass range with the same spin-parity assignments. As a result, I will briefly discuss these states from CLAS results of the single charged pion electroproduction data.},
doi = {10.1007/s00601-016-1145-6},
journal = {Few-Body Systems},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 8
}

Journal Article:
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  • The differential cross sections and structure functions for the exclusive electroproduction process ep --> e'n pi+ were measured in the range of the invariantmass for the np+ system 1.6 GeV lte W lte 2.0 GeV, and the photon virtuality 1.8 GeV2 lte Q2 lte 4.0 GeV2 using CLAS at Jefferson Lab. For the first time, these kinematics are probed in the exclusive p+ production from the protons with nearly full coverage in the azimuthal and polar angles of the np+ center-of-mass system. In this analysis, approximately 39,000 differential cross-section data points in terms of W, Q2, cosq theta* _ pi,more » and phi*_p-, were obtained. The preliminary differential cross section and structure function analyses are carried out, which allow us to extract the helicity amplitudes in high-lying resonances.« less
  • In this study, the transition helicity amplitudes from the proton ground state to the N(1440)1/2 +, N(1520)3/2 , and Δ(1620)1/2 resonances (γvpN* electrocouplings) were determined from the analysis of nine independent onefold differential π +π p electroproduction cross sections off a proton target, taken with CLAS at photon virtualities 0.5GeV 2 < Q 2 < 1.5 GeV 2. The phenomenological reaction model employed for separation of the resonant and nonresonant contributions to this exclusive channel was further developed.
  • Electroproduction of the Delta(1232) is a large component of a program at Jefferson Lab to study the Q{sup 2} evolution of the gammaNN* vertex for many of the low-lying nucleon resonances. Single pion electroproduction of the Delta provides information which is sensitive to models describing the internal motion of baryon constituents. Three multipoles contribute to the gamma*N -> transition: the electric, magnetic and scalar/longitudinal multipoles E1+, M1+ and S1+. At low Q{sup 2}, the focus has been on incorporating relativistic effects into the constituent quark model (CQM) as well as contributions from pion cloud and one gluon exchange. At highermore » Q{sup 2}, where the short distance behavior of the quark-gluon confining interaction is probed, QCD sum rule calculations, valence QCD and perturbative QCD (pQCD) have been employed. New measurements of the electroproduction of the Delta(1232) resonance through the p(e, e'p)pi{sup o} and p(e, e'pi+)n reactions have been performed. The data were taken with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab using incident electron energies of 1.645, 2.445 and 4.045 GeV. Elastic and inelastic cross sections were measured simultaneously with continuous coverage over a large range of four-momentum transfer Q{sup 2}=(0.3-4.0 GeV{sup 2}). Decay angular distributions in the piN center-of-mass were obtained over the full range of COS Thetac.m. and &oslash;c.m. The high statistical accuracy of this data set is expected to provide strong constraints on dynamic models of the N -> Delta transition form factors. A preliminary partial wave analysis will be presented.« less
  • We present results on the electroexcitation of the low mass resonances Delta(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive analysis of JLab-CLAS data on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for pion electroproduction off the proton. The data were analysed using two conceptually different approaches, fixed-t dispersion relations and a unitary isobar model, allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Delta(1232)P33} show the importance of a meson-cloud contribution to quantitatively explain themore » magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q2<6 GeV2. For the Roper resonance, N(1440)P11, the data provide strong evidence for this state as a predominantly radial excitation of a 3-quark ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S11 allowed us to obtain the branching ratios of this state to the piN and etaN channels via comparison to the results extracted from eta electroproduction. The extensive CLAS data also enabled the extraction of the gamma*p -> N(1520)D13 and N(1535)S11 longitudinal helicity amplitudes with good precision.« less