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.
Valence quark contributions for the gamma N -> P11(1440) form factors
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abstract
Using a covariant spectator quark model we estimate valence quark contributions to the F1*(Q2) and F2*(Q2) transition form factors for the gamma N -> P11(1440) reaction. The Roper resonance, P11(1440), is assumed to be the first radial excitation of the nucleon. The present model requires no extra parameters except for those already fixed by the previous studies for the nucleon. Our results are consistent with the experimental data in the high Q2 region, and those from lattice QCD. We also estimate the meson cloud contributions, focusing on the low Q2 region, where they are expected to be dominant.
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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.