First measurement of coherent $ϕ$-meson photoproduction from helium-4 near threshold
The differential cross sections and decay angular distributions for coherent $ϕ$-meson photoproduction from helium-4 have been measured for the first time at forward angles with linearly polarized photons in the energy range $E_γ = \text{1.685-2.385 GeV}$. Thanks to the target with spin-parity $J^{P} = 0^{+}$, unnatural-parity exchanges are prohibited, and thus natural-parity exchanges can be investigated clearly. The decay asymmetry with respect to photon polarization is shown to be very close to the maximal value. This ensures the dominance ($> 94\%$) of natural-parity exchanges in this reaction. To evaluate the contribution from natural-parity exchanges to the forward cross section ($θ= 0^\circ$) for the $γp \rightarrow ϕp$ reaction near the threshold, the energy dependence of the forward cross section ($θ= 0^\circ$) for the $γ{^{4}\text{He}} \rightarrow ϕ{^{4}\text{He}}$ reaction was analyzed. The comparison to $γp \rightarrow ϕp$ data suggests that enhancement of the forward cross section arising from natural-parity exchanges, and/or destructive interference between natural-parity and unnatural-parity exchanges is needed in the $γp \rightarrow ϕp$ reaction near the threshold.