First measurement of coherent $\phi$-meson photoproduction from helium-4 near threshold

TL;DR

This study presents the first measurement of coherent phi-meson photoproduction from helium-4 near threshold, revealing dominance of natural-parity exchanges and providing insights into reaction mechanisms at low energies.

Contribution

First experimental measurement of differential cross sections and decay distributions for coherent phi-meson photoproduction from helium-4 at near-threshold energies.

Findings

Natural-parity exchanges dominate (>94%) in the reaction.

Decay asymmetry indicates near-maximal polarization asymmetry.

Comparison suggests enhancement or interference effects in gamma p to phi p reactions.

Abstract

The differential cross sections and decay angular distributions for coherent $\phi$-meson photoproduction from helium-4 have been measured for the first time at forward angles with linearly polarized photons in the energy range $E_{\gamma} = \text{1.685-2.385 GeV}$. 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 ($\theta = 0^\circ$) for the $\gamma p \rightarrow \phi p$ reaction near the threshold, the energy dependence of the forward cross section ($\theta = 0^\circ$) for the $\gamma {^{4}\text{He}} \rightarrow \phi {^{4}\text{He}}$ reaction was analyzed. The comparison to $\gamma p \rightarrow \phi p$ 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 $\gamma p \rightarrow \phi p$ reaction near the threshold.