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

Abstract

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.