Abstract:
We provide an analysis of vector meson photoproduction in the channel of the vector meson decaying into a pseudoscalar meson plus a photon, i.e. $V\to P\gamma$. It is shown that non-trivial kinematic correlations arise from the measurement of the $P\gamma$ angular distributions in the overall c.m. system in comparison with those in the vector-meson-rest frame. In terms of the vector meson density matrix elements, the implication of such kinematic correlations in the measurement of polarization observables is discussed. For the $\omega$ meson production, due to its relatively large branching ratios for $\omega\to\pi^0\gamma$, additional events from this channel may enrich the information about the reaction mechanism and improve the statistics of the recent measurement of polarized beam asymmetries by the GRAAL Collaboration. For $\phi\to \eta\gamma$, $\rho\to \pi\gamma$, and $K^*\to K\gamma$, we expect that additional information about the spin structure of the vector meson production vertex can be derived.

Abstract:
An analysis of all possible polarization observables for the case of vector meson photoproduction from a nucleon target is presented. The question of which observables are needed to determine completely the basic photoproduction amplitudes and the relationships between spin observables are addressed.

Abstract:
A multipole analysis of vector meson photoproduction is formulated as a generalization of the pseudoscalar meson case. Expansion of spin observables in the multipole basis and behavior of these observables near threshold and resonances are examined.

Abstract:
We study the spin-dependent cross-sections of vector meson photoproduction for longitudinally and transversely polarized photons within a QCD- model. The dependence of the $\sigma_T/\sigma_L$ ratio on the photon virtuality and on the meson wave function is analysed.

Abstract:
Extraction of spin observables from vector meson photoproduction on a nucleon target is described. Starting from density matrix elements in the vector meson's rest frame, we transform to spin observables in the photon-nucleon c.m. frame. Several constraints on the transformed density matrix and on the spin observables follow from requiring that the angular distribution and the density matrix be positive definite. A set of constraints that are required in order to extract meaningful spin observables from forthcoming data are enunciated.

Abstract:
We discuss the photoproduction processes of light vector mesons ($\rho$, $\omega$, and $\phi$) from the nucleon near threshold. We first develop a simple model based on meson exchanges which is modified by the nucleon pole terms. We then extend this model to study other physically interesting topics. As examples, we discuss the missing nucleon resonances problem in $\omega$ photoproduction and the direct $\phi NN$ coupling constant in $\phi$ photoproduction. The calculated cross sections are compared with the recent experimental data. Various spin observables are discussed, which may be measured at current photon/electron facilities such as TJNAF and SPring-8 of RCNP. Precise measurements of such quantities would provide very useful information to understand the production mechanism.

Abstract:
The $\sigma$-exchange and $f_2$-exchange mechanisms for $\rho$ meson photoproduction are re-examined. Then the commonly employed $\sigma$-exchange amplitude is revised by using the recent information from the analyses on the $\rho \to \pi^0\pi^0\gamma$ decay and the $\sigma NN$ coupling constant from Bonn potential. Instead of relying on the Pomeron-$f$ proportionality assumption, the $f_2$ meson exchange amplitude is established from an effective Lagrangian which is constructed from the tensor structure of the $f_2$ meson. Phenomenological information together with tensor meson dominance and vector meson dominance assumptions are used to estimate the $f_2$ coupling constants. As a first step to improve the current theoretical models, we have also explored the effects due to the un-correlated $2\pi$ exchange amplitude with $\pi N$ intermediate state. This leading-order $2\pi$ exchange amplitude can be calculated using the coupling constants determined from the study of pion photoproduction and the empirical width of $\rho \to \pi\pi$ decay. In comparing with the existing differential cross section data, we find that a model with the constructed $2\pi$, $\sigma$, and $f_2$ exchanges is comparable to the commonly used $\sigma$ exchange model in which the $\sigma$ coupling parameters are simply adjusted to fit the experimental data. We suggest that experimental verifications of the predicted single and double spin asymmetries in the small $|t|$ ($ < 2$ GeV$^2$) region will be useful for distinguishing the two models and improving our understanding of the non-resonant amplitude of $\rho$ photoproduction. Possible further improvements of the model are discussed.

Abstract:
We present a quark model study of the $\omega$ meson photoproduction near threshold. With a limited number of parameters, all the data in history are reproduced. The roles played by the {\it s}- and {\it u}-channel processes (resonance excitations and nucleon pole terms), as well as the {\it t}-channel {\it natural} (Pomeron) and {\it unnatural} parity (pion) exchanges are clarified. This approach provides a framework for systematic study of vector meson photoproduction near threshold.

Abstract:
We considered the electro- and photoproduction of \rho and other vector mesons with moderately large transverse momentum in the scattering of unpolarized electrons on unpolarized nucleons and nuclei. For these processes we have analyzed how to extract the novel fragmentation functions \bar b_1^q(z), \bar b_1^G(z), describing tensor polarization, and the photon double spin-flip distribution \Delta^\gamma from the angular distribution of meson decay products.

Abstract:
We have studied in perturbative QCD all independent helicity amplitudes describing the photoproduction of light vector mesons at large $t$. We found a new hard production mechanism which is related to the possibility for a real photon to fluctuate into a massless $q\bar q$ pair in a chiral-odd spin configuration. Each helicity amplitude is given as a sum of a usual chiral-even contribution (when the helicities of quark and antiquark are antiparallel) and this additional chiral-odd part (where the helicities of quark and antiquark are parallel). The chiral-odd contribution is large, it leads to a dominance of the non spin-flip amplitude in a very broad region of intermediately high $|t|$. All amplitudes are expressed in terms of short distance asymptotics of the light-cone wave functions of vector meson (photon). We demonstrate that for each helicity amplitude there exists a soft non-factorizable contribution. We give arguments that for dominant non spin-flip helicity amplitude the relative contribution of the soft nonfactorizable interactions is numerically not large.