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

Unitarisation of the hard pomeron and black-disk limit at the LHC

Recent models of soft diffraction include a hard pomeron pole besides the usual soft term. Such models violate the black-disk limit around Tevatron energies, so that they need to be supplemented by a unitarisation scheme. Several such schemes are considered in this letter, where we show that they lead to a large uncertainty at the LHC. We also examine the impact of unitarisation on various small-t observables, the slope in t of the elastic cross section, or the ratio of the real to imaginary parts of the scattering amplitude, leading to the conclusion that the existence of a hard pomeron in soft scattering may be confirmed by LHC data.Comment: 15 pages, 12 figures, late

Systematic Regge theory analysis of omega photoproduction

Systematic analysis of available data for $\omega$-meson photoproduction is given in frame of Regge theory. At photon energies above 20 GeV the $\gamma{+}p{\to}\omega{+}p$ reaction is entirely dominated by Pomeron exchange. However, it was found that Pomeron exchange model can not reproduce the $\gamma{+}p{\to}\rho{+}p$ and $\gamma{+}p{\to}\omega{+}p$ data at high energies simultaneously with the same set of parameters. The comparison between $\rho$ and $\omega$ data indicates a large room for meson exchange contribution to $\omega$-meson photoproduction at low energies. It was found that at low energies the dominant contribution comes from $\pi$ and $f_2$-meson exchanges. There is smooth transition between the meson exchange model at low energies and Regge theory at high energies.Comment: 7 pages, 8 figures, revtex

Proton structure function at small Q^2

A fit is made to the data for the proton structure function up to Q^2=10 GeV^2, including the real gamma p total cross-section. It is economical and simple, and its form is motivated by physical principles. It is extrapolated down to very small values of x. Data for the ratio (nu W_2^n/nu W_2^p) are also fitted. A FORTRAN program for the fit to (nu W_2^p) is available by email on request Figure 5 from the original version has been deleted.Comment: 10 pages plus 9 figure

Rho Electroproduction and the Hadronic Contribution to Deeply Virtual Compton Scattering

A two-gluon-exchange model incorporating perturbative and non-perturbative effects is presented for rho electroproduction which provides an excellent description of all current data. This is then used to calculate the contribution from the rho to deeply virtual Compton scattering via the vector-meson-dominance transition rho -> gamma. This is found to be sufficiently large to provide a significant contribution through interference with the perturbative QCD term.Comment: 13 pages, 5 figures, 1 tabl

Gamma(*)Gamma(*) reaction at high energies

The energy available for gamma(*)gamma(*) physics at LEP2 is opening a new window on the study of diffractive phenomena, both non-perturbative and perturbative. We discuss some of the uncertainties and problems connected with the experimental measurements and their interpretation.Comment: 6 pages, 6 figures, submitted to proceedings of the Durham Collider Workshop, 22-26 September 199

Novel Color Transparency Effect: Scanning the Wave Function of Vector Mesons

We demonstrate how the virtual photoproduction of vector mesons on nuclei scans the wave function of vector mesons from the large non-perturbative transverse size $\rho \sim R_{V}$ down to the small perturbative size $\rho \sim 1/\sqrt{Q^{2}}$. Thee mechanism of scanning is based on color transparency and QCD predicted spatial wave function of quark-antiquark fluctuations of virtual photons. A rich, energy- and $Q^{2}$-dependent, pattern of the nuclear shadowing and antishadowing is predicted, which can be tested at the European Electron Facility and SLAC.Comment: TRI-PP-93-5, LaTeX file, 11 pages + 3 figures (not included, available by fax

Hadronic Phases and Isospin Amplitudes in D(B)→ππD(B) \to \pi \pi and D(B)→KKˉD(B) \to K \bar K Decays

Hadronic phase in $\pi \pi$ and $K \bar K$ channels are calculated a la Regge. At the D mass one finds $\delta_{\pi \pi} \approx \pi/3$ and $\delta_{K \bar K} \approx -\pi/6$ in good agreement with the CLEO data while at the B mass these angles are predicted to be, respectively, $11^\circ$ and $-7^\circ$. With the hadronic phase $e^{i \delta_{K \bar K}}$ taken into account, a quark diagram decomposition of the isospin invariant amplitudes in $D \to K \bar K$ decays fits the data provided the exchange diagram contribution is about 1/3 of the tree level one.Comment: 10pages,late

Photoproduction of Vector Mesons at Large Transfer

At forward angles, the cross-sections of photoproduction of vector mesons ($\rho$, $\omega$, and $\phi$) are well accounted for by the exchange of the Pomeron at high energies, while contributions of $t$ channel exchange of Reggeons are significant at low energies. At large angles, the impact parameter becomes small enough to prevent their constituents to build up the exchanged Reggeons or Pomeron. Two gluon exchange appears to dominate above $-t\simeq 1$ GeV$^2$, especially in the $\phi$ channel.Comment: 4 pages, 3 figure

Anomalous f_1 exchange in vector meson photoproduction asymmetries

We perform an analysis of the elastic production of vector mesons with polarized photon beams at high energy in order to investigate the validity of a recently proposed dynamical mechanism based on the dominance of the f_1 trajectory at large momentum transfer. The density matrix characterizing the angular distributions of the vector meson decays is calculated within an exchange model which includes the Pomeron and the f_1. The asymmetries of these decays turn out to be very useful to disentangle the role of these exchanges since their effect depends crucially on their quantum numbers which are different. The observables analyzed are accessible with present experimental facilities.Comment: 10 pages, REVTeX, 4 figures, some figures are corrected, conclusions unchange