Measurement of spin-density matrix elements for<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>ϕ</mml:mi></mml:mrow></mml:math>-meson photoproduction from protons and deuterons near threshold

Abstract

The LEPS/SPring-8 experiment made a comprehensive measurement of the\nspin-density matrix elements for $\\gamma p \\to \\phi p$, $\\gamma d \\to \\phi p n$\nand $\\gamma d \\to \\phi d$ at forward production angles. A linearly polarized\nphoton beam at $E_{\\gamma}$=1.6-2.4 GeV was used for the production of $\\phi$\nmesons. The natural-parity Pomeron exchange processes remains dominant\nnearthreshold. The unnatural-parity processes of pseudoscalar exchange is\nvisible in the production from nucleons but is greatly reduced in the coherent\nproduction from deuterons. There is no strong $E_{\\gamma}$-dependence, but some\ndependence on momentum-transfer. A small but finite value of the spin-density\nmatrix elements reflecting helicity-nonconserving amplitudes in the $t$-channel\nis observed.\n