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

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