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Lowest- Q2 measurement of the γp → Δ reaction: Probing the pionic contribution

  • The A1 Collaboration
  • S. Stave
  • M. O. Distler
  • I. Nakagawa
  • N. Sparveris
  • P. Achenbach
  • C. Ayerbe Gayoso
  • D. Baumann
  • J. Bernauer
  • A. M. Bernstein
  • R. Böhm
  • D. Bosnar
  • T. Botto
  • A. Christopoulou
  • D. Dale
  • M. Ding
  • L. Doria
  • J. Friedrich
  • A. Karabarbounis
  • M. Makek
  • H. Merkel
  • U. Müller
  • R. Neuhausen
  • L. Nungesser
  • C. N. Papanicolas
  • A. Piegsa
  • J. Pochodzalla
  • M. Potokar
  • M. Seimetz
  • S. Širca
  • S. Stiliaris
  • Th. Walcher
  • M. Weis
Open Access
Letter

Abstract.

To determine nonspherical angular-momentum amplitudes in hadrons at long ranges (low Q2), data were taken for the pe, e'p0 reaction in the Δ region at Q 2 = 0.060 (GeV/c)2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W = 1232 MeV are \(\ensuremath M_{1+}^{3/2}=(40.33 \pm 0.63_{\rm stat+syst} \pm 0.61_{\rm model})(10^{-3}/m_{\pi^+})\), Re( \(\ensuremath E_{1+}^{3/2}/M_{1+}^{3/2})=(-2.28 \pm 0.29_{\rm stat+syst} \pm 0.20_{\rm model}\))%, and Re( \(S_{1+}^{3/2}/M_{1+}^{3/2})\ensuremath =(-4.81 \pm 0.27_{\rm stat+syst} \pm 0.26_{\rm model}\))%. These disagree with predictions of constituent quark models but are in reasonable agreement with lattice calculations with nonlinear (chiral) pion mass extrapolations, with chiral effective field theory, and with dynamical models with pion cloud effects. These results confirm the dominance, and general Q2 variation, of the pionic contribution at large distances.

PACS.

13.60.Le Meson production 13.40.Gp Electromagnetic form factors 14.20.Gk Baryon resonances with S = 0 

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Copyright information

© The Author(s) 2006

Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.

Authors and Affiliations

  • The A1 Collaboration
  • S. Stave
    • 1
  • M. O. Distler
    • 2
  • I. Nakagawa
    • 1
    • 3
  • N. Sparveris
    • 4
  • P. Achenbach
    • 2
  • C. Ayerbe Gayoso
    • 2
  • D. Baumann
    • 2
  • J. Bernauer
    • 2
  • A. M. Bernstein
    • 1
  • R. Böhm
    • 2
  • D. Bosnar
    • 5
  • T. Botto
    • 1
  • A. Christopoulou
    • 4
  • D. Dale
    • 3
  • M. Ding
    • 2
  • L. Doria
    • 2
  • J. Friedrich
    • 2
  • A. Karabarbounis
    • 4
  • M. Makek
    • 5
  • H. Merkel
    • 2
  • U. Müller
    • 2
  • R. Neuhausen
    • 2
  • L. Nungesser
    • 2
  • C. N. Papanicolas
    • 4
  • A. Piegsa
    • 2
  • J. Pochodzalla
    • 2
  • M. Potokar
    • 6
  • M. Seimetz
    • 2
  • S. Širca
    • 6
  • S. Stiliaris
    • 4
  • Th. Walcher
    • 2
  • M. Weis
    • 2
  1. 1.Department of Physics, Laboratory for Nuclear Science and Bates Linear Accelerator CenterMassachusetts Institute of TechnologyCambridgeUSA
  2. 2.Institut für KernphysikJohannes Gutenberg-Universität MainzMainzGermany
  3. 3.Department of Physics and AstronomyUniversity of KentuckyLexingtonUSA
  4. 4.Institute of Accelerating Systems and Applications and Department of PhysicsUniversity of AthensAthensGreece
  5. 5.Department of PhysicsUniversity of ZagrebCroatia
  6. 6.Institute Jožef StefanUniversity of LjubljanaLjubljanaSlovenia

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