Eur. Phys. J. A (2006) 30: 471-476
https://doi.org/10.1140/epja/i2006-10162-1

Letter

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

¹ Department of Physics, Laboratory for Nuclear Science and Bates Linear Accelerator Center, Massachusetts Institute of Technology, 02139, Cambridge, MA, USA
² Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, D-55099, Mainz, Germany
³ Department of Physics and Astronomy, University of Kentucky, 40206, Lexington, KY, USA
⁴ Institute of Accelerating Systems and Applications and Department of Physics, University of Athens, Athens, Greece
⁵ Department of Physics, University of Zagreb, Croatia
⁶ Institute Jožef Stefan, University of Ljubljana, Ljubljana, Slovenia

Received: 21 September 2006
Accepted: 12 December 2006
Published online: 22 December 2006

Abstract

To determine nonspherical angular-momentum amplitudes in hadrons at long ranges (low Q²), data were taken for the p(¯e, e'p)π⁰ 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 , Re( )%, and Re( )%. 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.