arXiv subjects
Publications and source records attributed to M. Diehl.
The handbag contribution to Compton scattering at moderately large momentum transfer factorises into parton-photon subprocess amplitudes and new form factors representing 1/x-moments of skewed parton distributions. A detailed phenomenological study for polarised and unpolarised real and virtual Compton scattering is presented
Exclusive two-pion production near threshold in the collision of a highly virtual photon with a real one offers a new possibility to unravel the partonic content of hadrons. We explain the dynamics of this regime, i.e. the separation of the amplitude in terms of partonic diagrams computable in QCD perturbation theory and generalized distribution amplitudes, which are nonperturbative functions describing the exclusive transition of a parton pair into two hadrons. The same quantities also appear in gamma* N -> pi pi N in the kinematical regime where the momentum transfer of the nucleon and the invariant mass of the pion pair are both small; this allows to extend the QCD analysis of exclusive rho electroproduction outside the resonance region of the two produced pions.
We relate ordinary and skewed parton distributions to soft overlap contributions to elastic form factors and large angle Compton scattering, using light-cone wave functions in a Fock state expansion of the nucleon. With a simple ansatz for the wave functions of the three lowest Fock states we achieve a good description of unpolarised and polarised parton distributions at large x, and of the data for the Dirac form factor and for Compton scattering, both of which can be saturated with soft contributions only. Large angle Compton scattering appears as a good case to investigate the relative importance of soft and hard contributions in exclusive processes which are sensitive to the end point regions of the nucleon wave function.
We define and study hard ``semi-exclusive'' processes of the form $A+B \to C + Y$ which are characterized by a large momentum transfer between the particles $A$ and $C$ and a large rapidity gap between the final state particle $C$ and the inclusive system $Y$. Such reactions are in effect generalizations of deep inelastic lepton scattering, providing novel currents which probe specific quark distributions of the target $B$ at fixed momentum fraction. We give explicit expressions for photo- and leptoproduction cross sections such as $γp \to πY$ in terms of parton distributions in the proton and the pion distribution amplitude. Semi-exclusive processes provide opportunities to study fundamental issues in QCD, including odderon exchange and color transparency, and suggest new ways to measure spin-dependent parton distributions.
Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale.
We explain why the time ordering in the definition of non-diagonal parton distributions can be omitted and discuss the physics implications.
Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where h is a pion is emphasized.
The pomeron model of Donnachie and Landshoff does not conserve the electromagnetic current when applied to diffractive reactions such as electroproduction of a quark-antiquark pair or of a vector meson. We propose a treatment of this problem which ensures a physical behaviour of cross sections in the photoproduction limit and show that it leads to results rather similar to those obtained from two-gluon exchange.
We investigate the relevance of time ordering in the definition of off-diagonal parton distributions in terms of products of fields. The method we use easily allows determination of their support properties and provides a link to their interpretation from a parton point of view. It can also readily be applied to meson distribution amplitudes.}
We propose detailed tests of the handbag approximation in exclusive deeply virtual Compton scattering. Those tests make no use of any prejudice about parton correlations in the proton which are basically unknown objects and beyond the scope of perturbative QCD. Since important information on the proton substructure can be gained in the regime of light cone dominance we consider that such a class of tests is of special relevance.
We investigate azimuthal correlations in deep inelastic diffractive scattering, e + p -> e + \tilde{p} + X. The dependence of the ep cross section on the angle between the lepton plane and some direction in the hadronic final state can be written in a simple form; its measurement can be used to constrain the cross section for longitudinally polarised photons. Using the model of nonperturbative two-gluon exchange of Landshoff and Nachtmann we calculate the distribution of the azimuthal jet angle in diffractive dijet production and find that useful bounds on the longitudinal cross section for such events might be obtained from its measurement. We then discuss the predictions of this model for the dependence of the ep cross section on the azimuthal angle of the proton remnant \tilde{p}, which contains information about the helicity content of the pomeron.
The distribution of a suitably defined azimuthal angle in diffractive deep inelastic scattering contains information on the polarisation of the exchanged photon. In particular it allows one to constrain the longitudinal diffractive structure function. We investigate the potential of such bounds in general and for particular diffractive final states.
I review some recent developments in the QCD description of elastic vector meson production, focusing on issues like the meson wave function, the gluon density in the proton, factorisation, the use of parton-hadron duality and the nonperturbative vacuum.
We discuss the handbag approximation to exclusive deep virtual Compton scattering. After defining the kinematical region where this approximation can be valid, we propose tests for its relevance in planned electroproduction experiments, e + p -> e + p + gamma. We focus on scaling laws in the cross section, and the distribution in the angle between the lepton and hadron planes, which contains valuable information on the angular momentum structure of the Compton process. We advocate to measure weighted cross sections, which make use of the data in the full range of this angle and do not require very high event statistics.
We show how one can measure anomalous WWZ- and WWgamma-couplings with minimal statistical error using integrated observables, without having to assume that the anomalous couplings are small. We propose a parametrisation of these couplings which is well suited for the extraction of both single and many parameters, and which leads to a very simple form of the integrated cross section, from which additional information on the couplings can be obtained.
The cross section for the diffractive reaction gamma* + p -> c cbar + p with a real or virtual photon is calculated in the nonperturbative two-gluon exchange model of Landshoff and Nachtmann. Numerical predictions are given for cross sections and spectra at typical HERA values of c.m. energy and photon virtuality. The contribution of charm to the diffractive structure function is evaluated and found to be rather small in the model, and the ratio between the production rates for b bbar and c cbar is tiny.
We discuss the reaction $e p \to e \tilde{p} X$ in the one-photon exchange approximation, where it is in essence the reaction $γ^{*} p \to \tilde{p} X$. A large rapidity gap is required between the particle or particles of the proton remnant $\tilde{p}$ and those of $X$. We define a suitable azimuthal angle $φ$ between a leptonic and a hadronic plane. The dependence of the cross section on $φ$ is given explicitly and can be used to extract cross sections and interference terms for the reaction $γ^{*} p \to \tilde{p} X$ corresponding to the various helicities of the virtual photon $γ^{*}$. The interference terms can be used to test models for the large rapidity gap events in a sensitive way. We discuss in detail models with factorizing Pomeron exchange and in particular the Donnachie-Landshoff Pomeron model. We make some remarks on soft colour exchange models and on possible effects of QCD background vacuum fields. We conclude with a suggestion to look for Odderon exchange in exclusive deep inelastic high energy reactions like $γ^{*} p \to \tilde{p} π^{0}$ and $γ^{*} p \to \tilde{p} η$.
The critical exponents for $T\to0$ of the two-dimensional Ising spin glass model with Gaussian couplings are determined with the help of exact ground states for system sizes up to $L=50$ and by a Monte Carlo study of a pseudo-ferromagnetic order parameter. We obtain: for the stiffness exponent $y(=θ)=-0.281\pm0.002$, for the magnetic exponent $δ=1.48 \pm 0.01$ and for the chaos exponent $ζ=1.05\pm0.05$. From Monte Carlo simulations we get the thermal exponent $ν=3.6\pm0.2$. The scaling prediction $y=-1/ν$ is fulfilled within the error bars, whereas there is a disagreement with the relation $y=1-δ$.