Figure 1

Feynman diagrams for photon-induced pion production.Reuse & Permissions

Figure 2

The photoabsorption cross section on the free proton evaluated in terms of the isobar propagator of Ref. [16]. The data points are taken from Refs. [22, 23, 24].Reuse & Permissions

Figure 3

Feynman diagrams considered for the in-medium $\gamma \hspace{0.5em}N\to \pi \hspace{0.5em}N$ process.Reuse & Permissions

Figure 4

Photoabsorption cross section for variations of $g_{12}^{\text{'}}$ and $g_{22}^{\text{'}}$. We always keep ${\Sigma }_S^{\Delta }=-0.2$ GeV and $g_{11}^{\text{'}}=1.0$. The following values for ${\Sigma }_V^{\Delta }$ are used (corresponding to the legend from top to bottom): $-0.09,-0.10$, and −0.07 GeV (left panel); −0.09, −0.10, and −0.08 GeV (right panel). All parameters are for 0.8 times saturation density. The data are taken from Ref. [2].Reuse & Permissions

Figure 5

Photoabsorption cross section for $g_{11}^{\text{'}}=1.0$ (solid line) and $g_{11}^{\text{'}}=0.6$ (dashed line). We use ${\Sigma }_S^{\Delta }=-0.2$ GeV and ${\Sigma }_V^{\Delta }=-0.09$ GeV. In addition we show the background contribution for the run with $g_{11}^{\text{'}}=0.6$ (dash dotted line). The data are taken from Ref. [2].Reuse & Permissions

Figure 6

Photoabsorption cross section using the parameter set (20). The upper left panel shows the relevance of various contributions; the upper right panel shows the effect of short-range correlations in the $\gamma N\Delta$ vertex. The lower left panel follows if bare $\gamma N\Delta$ and $\pi N\Delta$ vertices are assumed. The solid lines give the complete calculations, the dashed lines the resonance contributions, the dashed-dotted lines the background contributions, and the dotted lines the two-particle-one-hole contributions. The lower right panel illustrates the importance of in-medium effects on the background processes. The short-dashed dotted line provides the background contribution evaluated with bare vertices and a free-space pion. The data are taken from Ref. [2]. See the text for more details.Reuse & Permissions