Valence quark contributions for the ${\gamma }^{*}N\to N(1440)$ form factors from light-front holography

The structure of the nucleon and the first radial excitation of the nucleon, the Roper, $N(1440)$, is studied within the formalism of light-front holography. The nucleon elastic form factors and ${\gamma }^{*}N\to N(1440)$ transition form factors are calculated under the assumption of the dominance of the valence quark degrees of freedom. Contrary to the previous studies, the bare parameters of the model associated with the valence quark are fixed by the empirical data for large momentum transfer ($Q^2$) assuming that the corrections to the three-quark picture (meson cloud contributions) are suppressed. The ${\gamma }^{*}N\to N(1440)$ transition form factors are then calculated without any adjustable parameters. Our estimates are compared with results from models based on valence quarks and others. The model compares well with the ${\gamma }^{*}N\to N(1440)$ transition form factor data, suggesting that meson cloud effects are not large, except in the region $Q^2<1.5{\mathrm{GeV}}^2$. In particular, the meson cloud contributions for the Pauli form factor are small.

Figure 1

Proton and neutron electric and magnetic form factors for the parametrization from Table 1. The red band represents the interval between parametrization with $Q_m^2=1.5{\mathrm{GeV}}^2$ and $2.5{\mathrm{GeV}}^2$. In the case of $G_{En}$, the dashed-line represents the parametrization with $Q_m^2=2.0{\mathrm{GeV}}^2$. Data from Refs. [89, 90, 91].

Figure 2

${\gamma }^{*}N\to N(1440)$ transition form factors $F_{1p}^{*}$ and $F_{2p}^{*}$. The red band represents the interval between the models with $Q_m^2=1.5{\mathrm{GeV}}^2$ and $2.5{\mathrm{GeV}}^2$. The dashed-line is the result from the light-front holographic model from Ref. [31]. Black dots from Ref. [92]; red dots from Ref. [93].