Photoproduction of pentaquark ${\Theta }^{+}$ and chiral symmetry restoration in a hot and dense medium

The photoproduction rate of pentaquark ${\Theta }^{+}$ is calculated in a hot and dense medium. At high temperature and density, photoproduction energy threshold is increased, due to the restoration of chiral symmetry. Above the threshold, the production cross section is strongly enhanced.

Figure 1

The diagrams for the process $\gamma n\to {\Theta }^{+}{K}^{-}$.

Figure 2

The pentaquark production cross section for the process $\gamma n\to {\Theta }^{+}{K}^{-}$ as a function of photon energy for six kinds of interactions, pseudoscalar, pseudovector, and hybrid couplings with positive and negative parity ${\Theta }^{+}$ (${\mathrm{PS}}^{+}$, ${\mathrm{PS}}^{-}$, ${\mathrm{PV}}^{+}$, ${\mathrm{PV}}^{-}$, ${\mathrm{HM}}^{+}$, and ${\mathrm{HM}}^{-}$). The solid, dotted, dashed and dot-dashed lines correspond, respectively, to the mediums with $(T,\mu )=(0,0)$, $(0.2,0.05)\mathrm{GeV}$, $(0.15,0.3)\mathrm{GeV}$, and $(0.1,0.8)\mathrm{GeV}$.

Figure 3

The cross section without considering the chiral symmetry restoration for the channel ${\mathrm{PS}}^{+}$. The left panel is the cross section around the threshold, and the right panel is the ratio of the cross section in the medium to the one in the vacuum. The solid, dotted, dashed, and dot-dashed lines correspond, respectively, to the mediums with $(T,\mu )=(0,0)$, $(0.2,0.05)\mathrm{GeV}$, $(0.15,0.3)\mathrm{GeV}$, and $(0.1,0.8)\mathrm{GeV}$.