Lowest Q² measurement of the [gamma] *p [right arrow] [Delta] reaction : probing the pionic contribution

Abstract

The first excited state of the proton, the Delta, can be reached through a magnetic dipole spin flip of one of the quarks (M1) or through electric and Coulomb quadrupole terms (E2 and C2) which indicate a deviation from spherical symmetry. The quark models using the color hyperfine interaction underestimate the size of the quadrupole terms by more than an order of magnitude. Models using the pion cloud do a much better job of describing the data. This is expected due to the spontaneous breaking of chiral symmetry which leads to a cloud of virtual p wave pions which introduce the non-spherical amplitudes. The data presented in this work fill gaps in the low Q2, long distance region where the pion cloud is expected to dominate and to produce significant Q2 variation. The p(e, e'p)7r0 reaction was measured in the A region at Q2 = 0.060 (GeV/c)2, the lowest Q2 to date for pion electroproduction, utilizing out-of-plane magnetic spectrometers at the Mainz Microtron in Germany.

(cont.) This work reports results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios obtained from fitting the new data with models using a three parameter, resonant multipole fit: M3/2 = (40.33 i 0.63stat+syst ± 0.61model) (10-3/m,r+), E2/M1 = Re(E3/2/M3/2) = (-2.28 i 0.29stat+syst ± 0.20modeI)%, and C2/M1 = Re(S3/M+3/2) = (-4.81 ± 0.27stat+syst i 0.26model)%. These new results for the transition multi-poles disagree with predictions of the quark models but are in reasonable agreement with a chiral extrapolation of lattice QCD, chiral effective field theory and dynamical model results confirming the dominance and general Q2 variation of the long range pionic contribution. While there is qualitative agreement with the models, there is no quantitative agreement thus indicating the need for further improvement of the models.