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Register a new userElectroexcitation of the $Delta^{+}(1232)$ at low momentum transfer
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We report on new p$(e,e^prime p)pi^circ$ measurements at the $Delta^{+}(1232)$ resonance at the low momentum transfer region. The mesonic cloud dynamics is predicted to be dominant and rapidly changing in this kinematic region offering a test bed for chiral effective field theory calculations. The new data explore the low $Q^2$ dependence of the resonant quadrupole amplitudes while extending the measurements of the Coulomb quadrupole amplitude to the lowest momentum transfer ever reached. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations that include pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.
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We report new p$(vec{e},e^prime p)pi^circ$ measurements in the $Delta^{+}(1232)$ resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.
$gamma p rightarrow K^+ Lambda$ differential cross sections and recoil polarisation data from threshold for extremely forward angles are presented. The measurements were performed at the BGOOD experiment at ELSA, utilising the high angular and momentum resolution forward spectrometer for charged particle identification. The data discriminates between conflicting results in the world data set and enable extraction of the cross section as the minimum momentum transfer to the recoiling hyperon is approached.
The reaction p(e,ep)pi^0 has been studied at Q^2=0.2 (GeV/c)^2 in the region of W=1232 MeV. From measurements left and right of q, cross section asymmetries rho_LT have been obtained in forward kinematics rho_LT(theta_pi^0=20deg) = (-11.68 +/- 2.36_stat +/- 2.36_sys)$ and backward kinematics rho_LT(theta_pi^0=160deg) =(12.18 +/- 0.27_stat +/- 0.82_sys). Multipole ratios Re(S_1+^* M_1+)/|M_1+|^2 and Re(S_0+^* M_1+)/|M_1+|^2 were determined in the framework of the MAID2003 model. The results are in agreement with older data. The unusally strong negative Re(S_0+^* M_1+)/|M_1+|^2 required to bring also the result of Kalleicher et al. in accordance with the rest of the data is almost excluded.
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%.
For the first time, the total yield and inclusive spectra of the $Delta^{++}(1232)$isobar are measured in $ u p$ and $ u n$ charged-current interactions. An indication is obtained that the $Delta^{++}(1232)$ production mainly results from the neutrino scattering on the valence d- quark of the target nucleon. The total yield of $Delta^{++}(1232)$ in $ u p$ interactions is compatible with that measured in hadronic interactions of the same net charge and net baryonic number. The yield of $Delta^{++}(1232)$ in $ u n$ interactions is significantly suppressed as compared to the case of the proton target. The form of the squared transverse momentum distributions, both in $ u p$ and $ u n$ interactions, is found to be compatible with the available data on the neutrinoproduction of $Lambda$ hyperon. The experimental data are compared with the LEPTO6.5 model predictions.
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