Electroexcitation of nucleon resonances
(Sep 7, 2011)Abstract
We review recent progress in the investigation of the electroexcitation of
nucleon resonances, both in experiment and in theory. The most accurate results
have been obtained for the electroexcitation amplitudes of the four lowest
excited states, which have been measured in a range of Q2 up to 8 and 4.5 GeV2
for the Delta(1232)P33, N(1535)S11 and N(1440)P11, N(1520)D13, respectively.
These results have been confronted with calculations based on lattice QCD,
large-Nc relations, perturbative QCD (pQCD), and QCD-inspired models. The
amplitudes for the Delta(1232) indicate large pion-cloud contributions at low
Q2 and don't show any sign of approaching the pQCD regime for Q2<7 GeV2.
Measured for the first time, the electroexcitation amplitudes of the Roper
resonance, N(1440)P11, provide strong evidence for this state as a
predominantly radial excitation of a three-quark (3q) ground state, with
additional non-3-quark contributions needed to describe the low Q2 behavior of
the amplitudes. The longitudinal transition amplitude for the N(1535)S11 was
determined and has become a challenge for quark models. Explanations may
require large meson-cloud contributions or alternative representations of this
state. The N(1520)D13 clearly shows the rapid changeover from helicity-3/2
dominance at the real photon point to helicity-1/2 dominance at Q2 > 0.5 GeV2,
confirming a long-standing prediction of the constituent quark model. The
interpretation of the moments of resonance transition form factors in terms of
transition transverse charge distributions in infinite momentum frame is
presented.