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Register a new userDiffractive photoproduction of vector mesons at large momentum transfer
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Diffractive photoproduction of rho, phi and J/psi was studied in the BFKL approach to hard colour singlet exchange. Differential cross sections, the energy dependence and spin density matrix elements were calculated and compared to data from HERA. The overall description of data is reasonably good, except of the single flip amplitude which has the wrong sign. Importance of chiral odd components of the photon is stressed.
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We consider the contribution to our understanding of vacuum-exchange processes to be made by investigations at the proposed electron-proton collider THERA. Recent results have highlighted the value of such studies for testing quantum chromodynamical descriptions of both long-range and short-range strong interactions. Stringent quantitative constraints have been provided by exploiting the opportunity to correlate scaling behaviour with helicity selection in exclusive and semi-exclusive vector-meson production. After reviewing the progress achieved by the measurement programs presently being carried out by the H1 and ZEUS collaborations at HERA, we discuss the performance criteria imposed by such investigations on the THERA accelerator complex and on the detector design. We conclude that the study of vector-meson production will form an essential component of the THERA physics program beginning with the early turn-on stage of the machine and continuing throughout the achievement of its full high-luminosity potential.
We discuss diffractive photon-production of vector mesons in holographic QCD. At large $sqrt{s}$, the QCD scattering amplitudes are reduced to the scattering of pair of dipoles exchanging a closed string or a pomeron. We use the holographic construction in AdS$_5$ to describe both the intrinsic dipole distribution in each hadron, and the pomeron exchange. Our results for the heavy meson photon-production are made explicit and compared to some existing experiments.
The collinear factorization framework allows to describe the exclusive photoproduction of a $gamma,rho$ pair in the generalized Bjorken regime in terms of a perturbatively calculable coefficient function and universal generalized parton distributions. The kinematics are defined by a large invariant mass of the $gamma rho$ pair and a small transverse momentum of the final nucleon. We calculate the scattering amplitude at leading order in $alpha_s$ and the differential cross sections for the process where the $rho-$meson is either longitudinally or transversely polarized, in the kinematics of the near future Jlab experiments. Our estimate of the cross section demonstrates that this process is measurable at JLab 12-GeV.
We apply perturbative QCD to investigate the near threshold heavy quarkonium photoproduction at large momentum transfer. From an explicit calculation, we show that the conventional power counting method will be modified and the three quark Fock state with nonzero orbital angular momentum dominates the near threshold production. It carries a power behavior of $1/(-t)^5$ for the differential cross section. We further comment on the impact of our results on the interpretation of the experiment measurement in terms of the gluonic gravitational form factors of the proton.
The differential cross section, $dsigma/dt$ for $omega$ meson exclusive photoproduction on the proton above the resonance region ($2.6<W<2.9$ GeV) was measured up to a momentum transfer $-t = 5$ GeV$^2$ using the CLAS detector at Jefferson Laboratory. The $omega$ channel was identified by detecting a proton and $pi^+$ in the final state and using the missing mass technique. While the low momentum transfer region shows the typical diffractive pattern expected from Pomeron and Reggeon exchange, at large $-t$ the differential cross section has a flat behavior. This feature can be explained by introducing quark interchange processes in addition to the QCD-inspired two-gluon exchange.
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