9 research outputs found
Deviation of the Nucleon Shape From Spherical Symmetry: Experimental Status
In this brief pedagogical overview the physical basis of the deviation of the
nucleon shape from spherical symmetry will be presented along with the
experimental methods used to determine it by the gamma* p -> Delta reaction.The
fact that significant non-spherical electric(E2) and Coulomb quadrupole(C2)
amplitudes have been observed will be demonstrated. These multipoles for the
N,Delta system as a function of Q^2 from the photon point through 4 GeV^2 have
been measured with modest precision. Their precise magnitude remains model
dependent due to the contributions of the background amplitudes, although rapid
progress is being made to reduce these uncertainties. A discussion of what is
required to perform a model independent analysis is presented. All of the data
to date are consistent with a prolate shape for the proton (larger at the
poles) and an oblate shape(flatter at the poles) for the Delta. It is suggested
here that the fundamental reason for this lies in the spontaneous breaking of
chiral symmetry in QCD and the resulting, long range(low Q^2), effects of the
pion cloud. This verification of this suggestion, as well as a more accurate
measurement of the deviation from spherical symmetry, requires further
experimental and theoretical effort.Comment: 8 pages, 8 figures, enhanced conference proceeding