Including destructively interfering off-diagonal transitions of diffraction-dissociation type, we arrive at a formulation of GVD for exclusive vector-meson production in terms of a continuous spectral representation of dipole form. The transverse cross-section, σT,γ∗p→Vp, behaves asymptotically as 1/Q4, while RV≡σL,γ∗p→Vp/σT,γ∗p→Vp becomes asymptotically constant. Contributions violating s-channel helicity conservation stay at the level established in low-energy photoproduction and diffractive hadron–hadron interactions. The data on ρ0-meson production from the Fermilab E665 Collaboration and on φ- and ρ0-meson production from HERA are found to be in agreement with these predictions.

Meson-photon-photon transition form factors for S-, P-, and D-wave states are calculated, the meson being treated as a non-relativistic heavy-quark-antiquark pair. The full dependence on both photon virtualities is included. Cross-section formulas for charge-conjugation even mesons with J(P) = 0(-), 0(+), 1(+), 2(+), and 2(-) in electron-positron collisions are presented and numerical results for LEP energies are given. In particular, we find two-photon event rates for chi(cl) eta(c) (2S), and eta(b) (1S) within reach of LEP.With minor modifications to incorporate SU(3)-flavour breaking we estimate rates for 18 light mesons as well, based on the observation that their two-photon decay widths agree remarkably well with measured data. Finally we point out that e(+)e(-) cross sections for 1(+) states do not vanish at low Q(2), the Landau-Yang suppression factors of the two-photon cross sections being compensated by the photon propagators. (C) 1998 Elsevier Science B.V.

Recent progress in the factorization of exclusive decays of heavy quarkonia into long-distance and short-distance contributions is presented. The role of higher Fock states and colour-octet contributions is outlined.

An extended version of the Monte Carlo program GALUGA is presented for the computation of two-photon production in $\e^+\e^-$ collisions. Functions implemented for the five $\gamma^\star\gamma^\star$ structure functions now include several ans\"atze of the total hadronic cross section based on the BFKL--Pomeron and various Regge-like models. In addition, structure functions for resonance formation are included with full dependence on the two photon virtualities $Q_1^2$ and $Q_2^2$ as given in the constituent-quark model. The six lowest-lying resonances of each of the C-even mesons with $J^{P}= 0^-$, $0^+$, $1^+$, $2^+$ and $2^-$ are provided. The program can also be used to calculate with exact kinematics the effective two-photon luminosity function. Special emphasis is put on a numerically stable evaluation of all variables over the full $Q_i^2$ range while keeping all dependences on the electron mass and $Q_i^2$.

The hierarchy of long-distance matrix elements (MEs) for quarkonium production depends on their scaling with the velocity v of the heavy quark in the bound state. Ranges for the velocities in various bound states and uncertainties of colour-singlet MEs are estimated in a quark-potential model. Different possibilities for the scaling with v of the MEs are discussed; they depend on the actual values of v and the QCD scale. As an application, J/ψ polarization in e+e- annihilation is discussed. The first non-perturbative estimates of colour-octet MEs are presented and compared with phenomenological determinations. Finally, various predictions of prompt quarkonium production at LEP are compared.

We investigate the theoretical uncertainties of P-wave charmonium decays into two pions, χcJ → π+π−, π0π0. Constraining the pion distribution-amplitude from the recent precise data on Fπγ(Q2), we find an order-of-magnitude discrepancy between data and prediction. The disagreement persists even after inclusion of transverse degrees of freedom and Sudakov suppressions. We propose the colour-octet mechanism as the solution to the puzzle. The colour-octet decay contribution arising from the higher Fock component |ccg〉 of the χcJ wave function is actually not power suppressed with respect to the usual colour-singlet decay arising from the dominant |cc〉 Fock state. An explicit calculation yields an agreement with the data for a very reasonable value for the single extra non-perturbative parameter.

A program to simulate the production of heavy quarks through the boson-gluon fusion process in e(+/-)p collisions is presented. The full electroweak structure of the electron-gluon interaction is taken into account as well as the masses of the produced heavy quarks. Higher order QCD radiation is treated using initial and final state parton showers, and hadronization is performed using the Lund string model, Physics and programming aspects are described in this manual.

Quarkonium production in high-energy reactions is found to exhibit a behaviour more universal than that expected from velocity scaling. Total rates of quarkonia produced in hadronic interactions as well as Feynman-x and transverse momentum distributions can be described over the full range of accessible energies (15 ≲ s ≲ 1800 GeV) by two-stage processes. The quarkonium production cross section factors into a process-dependent short-distance part and a single long-distance matrix element. The first part describing the production of a free quark-antiquark pair is the perturbatively calculated subthreshold cross section. The non-perturbative factor turns out to be universal, giving the model great predictive power. Furthermore we estimate the fraction of the heavy-quark cross section leading to quarkonium for both the charm and bottom systems. Finally, we comment on quarkonium photoproduction.

We propose a generic ansatz for the extension of parton distributions of the real photon to those of the virtual photon. Alternatives and approximations are studied that allow closed-form parametrizations.

This report is an overview of the gamma-gamma physics capabilities of LEP2, and covers the following topics: structure functions, equivalent photon approximation, tagging conditions etc, soft and semihard physics, large-$p_t$ processes, heavy-quark physics, and exclusive channels.

We investigate the effects of photon radiation on deep-inelastic e gamma scattering. Depending on the set of variables chosen, we find appreciable effects in the kinematic region accessible at LEP2. Convenient analytic results for O(alpha) corrected differential cross sections are presented.

We discuss systems containing a heavy quark and a heavy antiquark in the infinite mass limit of QCD. Studying the limit of equal velocities for both heavy particles, we formulate an effective theory approach to heavy quarkonia-like systems. The method is well suited to processes in which the two heavy quarks annihilate, such as electromagnetic and strong decays of charmonium and bottomonium and weak decays of Bc.

A real photon has a dual nature, as a pointlike elementary particle and as a qq fluctuation, in the latter case often with hadron-like properties. Therefore high-energy γp interactions have a rich and not yet well explored structure. In this paper, we propose a consistent formulation, which is based on a subdivision of the total cross section into three distinct event classes: direct, VMD and anomalous. The VMD class, in its turn, contains the subclasses familiar from hadron-hadron physics. Within this approach, we calculate all the partial cross sections. Many other aspects of the event classes are also considered, such as proton and photon structure functions, jet rates and beam remnant structures. A major rôle is played by two distinct transverse momentum cut-off scales, which separate the classes. When all the ingredients are put together, we obtain a detailed model for photoproduction events. In order to test this approach, we also propose several alternative scenarios, each of which emphasizes different aspects of the photon. Predictions are presented for HERA events.

We propose a consistent formulation of high-energy γp interactions, in which these are built up as a sum of three contributions. A significant role is played by the hadronic structure of the photon. From the total cross section at low energy we determine the relative sizes of the hadronic contribution and of the components that make a photon distinctly different from a vector-meson, i.e. direct and anomalous photon components. We then obtain predictions for γp collisions at higher energies. Special attention is paid to semi-hard cross sections (mini-jets) and the origin of constituents in the photon.

We discuss photo- and leptoproduction of J/psi mesons at energies ranging from those of fixed-target experiments up to those of HERA. Elastic and diffractive production as well as various inelastic processes are studied. We investigate the range in which J/psi production is described by photon-gluon fusion in the color-singlet model. We show how inelastic J/psi production at HERA can be used to extract the gluon density. We estimate an accessible range of 3 X 10(-4) < x < 0.1 and discuss sources of errors in the reconstruction of the gluon density at HERA.

The large polarization recently reported by Fermilab experiment E691 in D→K∗lν casts doubt on quark model from factors. We investigate whether unitarity corrections can make the standard quark model consistent with the E691 data. Despite significant corrections, we show that the quark model still cannot achieve the high polarization of the E691 data. Unitarity corrections, including CP-odd effects, will be visible in the next generation experiments.

We report first results on O(αs2) corrections to the parity conserving structure functions that describe high-qT polarized gauge boson production in hadron collisions. We present some numerical results for polarized W+ production at the Tevatron pp collider. In particular we find that the relation A0=A2 between the longitudinal and transverse interference structure functions no longer holds true at next-to-leading order (NLO). We calculate the NLO corrections to this relation.

The order αs3 QCD corrections to the parton-parton cross sections contributing to heavy flavour production in hadron-hadron collisions are discussed. We construct simple formulae which should yield reasonable approximations to the exact order αs3 results. Then we examine the differences between the predictions of the exact and approximate formulae in the case of the reaction p + p → Q + Q + X where Q is either the b or the t quark.