Extracting meson-baryon contributions to the electroexcitation of the $N\left(1675\right){\frac{5}{2}}^{-}$ nucleon resonance

We report on the determination of the electrocouplings for the transition from the proton to the $N\left(1675\right){\frac{5}{2}}^{-}$ resonance state using recent differential cross section data on $ep\to e{\pi }^{+}n$ by the CLAS collaboration at $1.8\le Q^2<4.5$ ${\mathrm{GeV}}^2$. The data have been analyzed using two different approaches, the unitary isobar model and fixed-$t$ dispersion relations. The extracted ${\gamma }^{*}p\to N\left(1675\right){\frac{5}{2}}^{-}$ helicity amplitudes show considerable coupling through the $A_{1/2}^p$ amplitude, that is significantly larger than the predicted three-quark contribution to this amplitude. The amplitude $A_{3/2}^p$ is much smaller. Both results are consistent with the predicted sizes of the meson-baryon contributions at $Q^2\ge 1.8$ ${\mathrm{GeV}}^2$ from the dynamical coupled-channel model.

Figure 1

Symbolic representation of the main contributions to the ${\gamma }^{*}N\to N^{*}$ transition: (a) through quark transition; (b,c) through meson-baryon pairs.

Figure 2

Differential cross sections for the ${\gamma }^{*}p\to n{\pi }^{+}$ reaction at $W=1.675$ GeV as a function of pion azimuthal angle $\varphi$ at different values of pion polar angle $\theta$. Data are from Ref. [24]. The rows correspond to $Q^2=1.8$ ${\mathrm{GeV}}^2$, $\epsilon =0.866$; $Q^2=2.6$ ${\mathrm{GeV}}^2$, $\epsilon =0.792$; $Q^2=4$ ${\mathrm{GeV}}^2$, $\epsilon =0.626$. The columns correspond to $\cos \theta =-0.5,0.1,0.5$. The solid (dashed) curves are the results obtained using UIM (DR) approach.

Figure 3

Transverse helicity amplitudes for the ${\gamma }^{*}p\to N\left(1675\right){\frac{5}{2}}^{-}$ transition. The full circles are the amplitudes extracted from the data [24, 25] using the following mass, width, and $\pi N$ branching ratio of the resonance: $M=1.675$ GeV, $\mathrm{\Gamma }=0.15$ GeV, and ${\beta }_{\pi N}=0.4$. The bands show the model uncertainties. The full triangles at $Q^2=0$ are the RPP estimates [5]. The solid and dashed curves correspond to quark model predictions of Refs. [22] and [23], respectively; the dots at $Q^2=0$ are the predictions of the light-front relativistic quark model from Ref. [21]. Dashed-dotted curves are absolute values of the predicted meson-baryon contributions from the dynamical coupled-channel model of Ref. [13].