3 years ago

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

T. Nakano, M. Oka, W. C. Chang, T. Hiraiwa, S. Daté, Y. Sugaya, Y. Ohashi, T. Hotta, H. Kohri, S. Fukui, H. Fujimura, D. S. Ahn, Y. Nozawa, Y. Maeda, A. O. Tokiyasu, S. Y. Ryu, LEPS Collaboration, Y. Kon, T. Ohta, T. Saito, N. Muramatsu, H. Kawai, K. Mizutani, C. Rangacharyulu, J. K. Ahn, Y. Kato, K. Hicks, K. Suzuki, T. Ishikawa, M. Yosoi, T. Tsunemi, T. Tomioka, Y. Morino, T. Yorita, M. Niiyama, H. Shimizu, T. Sawada, M. Uchida, M. Sumihama, Y. Nakatsugawa, M. Miyabe, E. A. Strokovsky, J. Y. Chen, P. J. Lin, S. H. Hwang

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 (%CONTENT%gt; 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.

Publisher URL: http://arxiv.org/abs/1711.01095

DOI: arXiv:1711.01095v3

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