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Electromagnetic Properties of the Delta(1232)
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We calculate the electromagnetic moments and radii of the Delta(1232) in the nonrelativistic quark model, including two-body exchange currents. We show that two-body exchange currents lead to nonvanishing Delta and N-->Delta transition quadrupole moments even if the wave functions have no D-state admixture. The usual explanation based on the single-quark transition model involves D-state admixtures but no exchange currents. We derive a parameter- free relation between the N-->Delta transition quadrupole moment and the neutron charge radius: Q(N-->Delta) = r^2(neutron)/sqrt(2). Furthermore, we calculate the M1 and E2 amplitudes for the process photon + N -->Delta. We find that the E2 amplitude receives sizeable contributions from exchange currents. These are more important than the ones which result from D-state admixtures due to tensor forces between quarks if a reasonable quark core radius of about 0.6 fm is used. We obtain a ratio of E2/M1=-3.5%.
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Cited by 1 Pith paper
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Low-$Q^2$ empirical parametrizations of the $N^\ast$ helicity amplitudes
Empirical helicity amplitude fits for nine nucleon and Delta resonances are modified near the pseudothreshold using a polynomial in the photon momentum matched at QP^2 = 0.1, 0.3, and 0.5 GeV^2.
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