NEW LOW MOMENTUM-TRANSFER MEASUREMENTS OF THE p(e,e'p)pi0 CROSS SECTION TO ACCESS THE COULOMB QUADRUPOLE AMPLITUDE OF THE Delta (1232) RESONANCE
Deviation of the proton's wave function from pure spherical symmetry, referred to as proton deformation, has been a topic of continued interest for the last few decades, as its examination may lead to be a better understanding of proton structure, especially with regard to the roles of virtual pions in that structure and their interplay with the quark core. In the low momentum transfer region, this deformation can be explored by examining the electromagnetic transitions between the proton and the Delta(1232). In March 2011, Experiment E08-010 in Hall A at Jefferson Lab measured p(e,e'p)pi0 reaction cross sections with energies near the Delta(1232) resonance. Measurements were made at three low momentum transfer values between the electron and proton, Q2 = 0.045, 0.090, and 0.125 (GeV/c)2, using an 1160 MeV electron beam and Hall A's two high resolution spectrometers. Each momentum transfer value included multiple cross section measurements to exploit the dependence of the recoil proton's polar and azimuthal angles on the cross section. The goal of these measurements was to extract the magnetic dipole and the Coulomb quadrupole transition amplitudes, as well as their ratio, the CMR, in a region sensitive to the effects of the pion cloud contribution. These values were extracted from the cross sections using a model-independent multipole extraction method. The results were compared to model predictions, which found reasonable agreement for the two higher momentum transfer values, where previous measurements had previously existed, but none of the models were able to properly predict the behavior of the momentum transfer dependency of the data. These measurements thus provide new experimental impetus for the current theoretical models to revisit how the pion cloud dynamics influence proton deformation in this particularly sensitive low momentum transfer region.