\(\phi \) Meson Photoproduction on the Nucleon and \({}^4\)He Targets

We study the reaction mechanism of \(\phi \) $\varphi$ -meson photoproduction on the nucleon and \({}^4\) ${}^4$ He targets by using a dynamical model based on a Hamiltonian. In addition to the dominant contribution of the Pomeron exchange, various meson exchanges are considered in the t channel to describe the CLAS data in the low energy region \({\sqrt{s}} = (1.97 - 2.84)\) $\sqrt{s}=(1.97-2.84)$ GeV. The direct \(\phi \) $\varphi$ radiations are taken into account in the s- and u-channels. The backward structures at \({\sqrt{s}} \approx 2.1\) $\sqrt{s}\approx 2.1$ and 2.3 GeV are well reproduced by the inclusion of the s-channel \(N(2000,5/2^+)\) $N(2000,5/2^{+})$ and \(N(2300,1/2^+)\) $N(2300,1/2^{+})$ resonances, respectively. We also consider the final \(\phi N\) $\varphi N$ interactions by the gluon exchange, the direct \(\phi N\) $\varphi N$ interactions, and the box diagrams arising from the couplings with the \(\pi N\) $\pi N$ , \(\rho N\) $\rho N$ , \(K \Lambda \) $K\mathrm{\Lambda }$ , and \(K \Sigma \) $K\mathrm{\Sigma }$ channels. The effects of the final state interactions are found to be very weak. Then the resulting Hamiltonian is used to study the coherent \(\gamma {}^4{\textrm{He}} \rightarrow \phi {}^4{\textrm{He}}\) $\gamma {}^4\text{He}\to \varphi {}^4\text{He}$ reaction within the distorted-weave impulse approximation. The calculated differential cross sections account for the LEPS data quite well.