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Abstract
The helicity structure of the diffractive reaction γ* + A → [special characters omitted] + A was studied with [special characters omitted] mesons produced by longitudinally polarized positrons scattering from nuclear targets. By moment analysis of the distributions of data in the azimuthal production angle and two [special characters omitted] → K+K −decay angles, values for the elements of the spin-density matrix of the [special characters omitted] meson were measured. As the helicity structure of the photon is known from the theory of Quantum Electrodynamics, the target contribution to the helicity of the [special characters omitted] meson can be inferred. The data indicate that the [special characters omitted] meson preserves the helicity of the photon in the kinematic range of this experiment, with the magnitude of the helicity preserving amplitude measured to be |T11| = 1.28 ± 0.05 and the magnitude of the largest helicity changing amplitude measured to be | T1−1| = 0.36 ± 0.14. The predictions of a 2g exchange model based on perturbative Quantum Chromodynamics are compared to the data, and found not to agree with the observed matrix elements.
With the observation that the helicity of the photon is preserved by the [special characters omitted], the ratio of the longitudinal to transverse polarized photoproduction cross sections for the [special characters omitted] meson, [special characters omitted] = &sgr;L /&sgr;T, can be computed from the measured longitudinal polarization of the [special characters omitted]. The data are combined with previous published data, and fit to the parameterization [special characters omitted] = (0.38 ± 0.04) (Q2/[special characters omitted])0.87±012, in approximate agreement with the Vector Meson Dominance model of the photon. The results are compared with the results for the ρ0 vector meson.