Exclusive ηc\eta_c photo- and electroproduction at HERA as a possible probe of the odderon singularity in QCD

Theory and phenomenology of the eta_c photo- and electroproduction is developed from the point of view of probing the odderon singularity in QCD which corresponds to the three gluon exchange mechanism. This mechanism leads to the cross-sections which are independent of W^2 for the exchange of three non-interacting gluons or exhibit increase with increasing W^2 (or 1/x) for the odderon intercept above unity. The \eta_c electroproduction in the three gluon exchange mechanism is shown to be entirely controlled by the transversely polarised virtual photons. The magnitude of the eta_c photoproduction cross-section is estimated to be around 11-45pb. The t-dependence of the differential cross-section is also discussed.Comment: 9 pages, LaTeX, uses epsfig.sty; Numerical error corrected. A factor of 1/3 missing in amplitude in Eq.(8) added. Obtained gamma p -> eta_c p cross-section smaller by a factor of 9 than in the original version of the paper. Figures 4 and 5 changed accordingl

The QCD pomeron at TESLA - motivation and exclusive J/psi production

We briefly present the motivation for studying the processes mediated by the QCD pomeron at high energy e+e- colliders. We describe the behaviour of the cross-section for the reaction gamma gamma to J/psi J/psi obtained from the BFKL equation with dominant non-leading corrections. We give the predictions for the rates of anti-tagged e+e- to e+e- J/psi J/psi events in TESLA and conclude that such reactions may be excellent probes of the hard pomeron.Comment: 8 pages, 3 figures, to appear in the Proceedings of the 2nd Joint ECFA/DESY Study on Physics and Detectors for a Linear Electron-Positron Collider, Obernai, France, October 199

Probing the QCD pomeron in doubly tagged e^+ e^- collisions

We calculate the total cross-section for $\gamma^* \gamma^*$ collisions and for the process $e^+ e^- \to e^+ e^- + hadrons$ with two tagged leptons assuming dominance of the QCD pomeron exchange. We solve the BFKL equation including the dominant subleading effects generated by the consistency constraint which restricts the available phase-space of the emitted gluons to the region inwhich the virtuality of the exchanged gluons is dominated by their transverse momentum squared. Estimate of the possible soft pomeron contribution to the $\gamma^* \gamma^*$ cross-section is also presented. At very high CM energies $W$ the calculated total $\gamma^* \gamma^*$ cross-section exhibits effective power-law $(W^2)^{\lambda}$ behaviour with $\lambda \sim 0.3$. We confront our results with the recent measurements at LEP and give predictions for the energies which can be accessible at TESLA and at other future linear $e^+ e^-$ colliders.Comment: 12 pages, LaTeX, 3 figure

QCD at Photon Colliders

The novel possibilities of probing the photon structure and high energy limit of QCD at photon colliders are summarised. We discuss the photon structure function $F_2^{\gamma}(x,Q^2)$, the gluon distribution in the photon and the spin dependent structure function $g_1^{\gamma}(x,Q^2)$ of the photon and emphasise advantages of the photon colliders for measuring these quantities. The possibility of probing the QCD pomeron and odderon in $\gamma\gamma$ and $e \gamma$ collisions is also described.Comment: 14 pages with 3 postscript figures included, introductory talk given at the International Workshop on High Energy Photon Colliders, 14th - 17th June 2000, DESY, Hamburg, German