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

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 $\ensuremath M_{1+}^{3/2}=(40.33 \pm 0.63_{\rm stat+syst} \pm 0.61_{\rm model})(10^{-3}/m_{\pi^+})$ , 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.