Measurement of Spin-Density Matrix Elements for $\ddot{I}•$-Meson Photoproduction from Protons and Deuterons Near Threshold

Abstract

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for ${\hat{I}}^{3}p\to \ddot{I}•p$, ${\hat{I}}^{3}d\to \ddot{I}•pn$ and ${\hat{I}}^{3}d\to \ddot{I}•d$ at forward production angles. A linearly polarized photon beam at $E_{\hat{I}}^3$=1.6-2.4 GeV was used for the production of $\ddot{I}•$ mesons. The natural-parity Pomeron exchange processes remains dominant nearthreshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from nucleons but is greatly reduced in the coherent production from deuterons. There is no strong $E_{\hat{I}}^3$-dependence, but some dependence on momentum-transfer. A small but finite value of the spin-density matrix elements reflecting helicity-nonconserving amplitudes in the $t$-channel is observed.

25 pages, 14 figures