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Pion cloud and the $Q^2-$dependence of $\gamma^* N \leftrightarrow \Delta$ transition form factors
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abstract
Recent experiment indicates that the behavior exhibited by the ratios $E_{1^+}/M_{1^+}$ and $S_{1^+}/M_{1^+}$ of the $\gamma^* N \leftrightarrow \Delta$ transition remain small and negative for $Q^2 \le 4.0 GeV^2$. It implies that the perturbative QCD is still not applicable at these momentum transfers. We show that these data can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare $\gamma^* N \Delta$ form factors. Within our model we find that the bare $\Delta$ is almost spherical and the electric E2 and Coulomb C2 quardrupole excitations of the physical $\Delta$ are nearly saturated by pion cloud contribution in $Q^2 \le 4.0 GeV^2$.
Forward citations
Cited by 2 Pith papers
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Multipole structure of the $N \to \Delta$ Transition Generalized Parton Distributions
The four N oΔ transition GPDs decompose into monopole, two dipole and quadrupole multipole GPDs in one-to-one correspondence with light-front helicity amplitudes, defining impact-parameter transition densities at zero...
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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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