Direct extraction of hadronic form factors from elastic-scattering data

Non-forward elastic hadron-scattering data are collected and analysed within the Regge approach. Through an analysis of the data in small bins in $t$, we have directly extracted the pomeron trajectory and the hadronic form factors (or reggeon couplings). We found higher values than usually used for the intercept and for the slope of the soft pomeron trajectory. The presence of zeros in $t$ for the effective hadronic form factors is emphasised.Comment: 4 pages, 5 figures, talk given at Diffraction 2004, Cala Gonone, Sardinia, Italy, 18-23 September 200

Saturation effects in pp scattering in the impact-parameter representation

The impact of unitarity is considered in different approaches to saturation in impact-parameter space. The energy and momentum-transfer dependence of the total and differential cross sections and of the ratio of the real to imaginary parts of the scattering amplitude are obtained in a model that includes soft and hard pomeron contributions, coupled to hadrons via the electromagnetic form factor. It is shown that the hard pomeron may significantly contribute to soft physics at the LHC. A similar conclusion can also be reached in the framework of non-linear approaches to unitarisation of the BFKL pomeron.Comment: 3 pages, 1 figure, uses espcrc2.sty. Contribution to ``Diffraction 2004'', International Workshop on Diffraction in High-Energy Physics, Cala Gonone, Sardinia, September 18-23, 200

Large pion pole in Z_{S}^{MOM}/Z_{P}^{MOM} from Wilson action data

We show that, contrarily to recent claims, data from the Wilson (unimproved) fermionic action at three different beta values demonstrate the presence of a large Goldstone boson contribution in the quark pseudoscalar vertex, quantitatively close to our previous estimate based on the SW action with c_{SW}=1.769. We show that discretisation errors on Z_{S}^{MOM}/Z_{P}^{MOM} seem to be much smaller than the Goldstone pole contribution over a very large range of momenta. The subtraction of this non perturbative contribution leads to numbers close to one-loop BPT.Comment: 12 pages, 5 figures, laTeX, minor corrections of typos, beta dependence made more explicit, added one table giving the contribution of the Goldstone vs. the discretisation errors at ap=

Does F2 need a hard pomeron?

We show that the latest HERA measurements of F2 are compatible with the existence of a triple pole at J=1. Such a structure also accounts for soft forward hadronic amplitudes, so that the introduction of a new singularity is not necessary to describe DIS data.Comment: 11 pages, 4 figures, minor typos in the number of data points and in the definition of b fixed, references clarifie

Analytic Amplitudes for Hadronic Forward Scattering : COMPETE Update

We consider several classes of analytic parametrizations of hadronic scattering amplitudes, and compare their predictions to all available forward data in hadron-hadron, gamma-p and gamma-gamma reactions. Although these parametrizations are very close for SQRTs larger than 9 GeV, it turns out that they differ markedly at low energy, where a universal Pomeron term like ln**2 s enables one to extend the fit down to SQRTs equal to 4 GeV. We present predictions on the total cross sections and on the ratio of the real part to the imaginary part of the elastic amplitude (RHO parameter) for present and future pp and antipp colliders, and on total cross sections for gamma-p into hadrons at cosmic-ray energies and for gamma-gamma into hadrons up to SQRTs equal to 1 TeV.Comment: 3 pages, 3 tables. To be published in the Proceedings of the 31st International Conference on High Energy Physics (ICHEP 2002), Amsterdam, The Netherlands, 24-31 July 200

Vector Meson Production at HERA

We show that the lowest-order QCD calculation in a simple model of elastic vector-meson production does reproduce correctly the ratios of cross sections for rho, phi and J/psi, both in photoproduction and in high-Q2 quasi-elastic scattering. The dependence of the slopes on the mass of the vector meson is reproduced as well. We examine the lower-energy data, and find that the energy dependence of the cross section does not depend on Q2, but may depend on the vector-meson mass.Comment: 12 pages, Latex, 6 figures. Shortened version of the previous paper, which also includes a clearer criticism of the work by Martin, Ryskin and Teubner, hep-ph/960944

The Quark Propagator from the Dyson-Schwinger Equations: I. the Chiral Solution

Within the framework of the Dyson-Schwinger equations in the axial gauge, we study the effect that non-perturbative glue has on the quark propagator. We show that Ward-Takahashi identities, combined with the requirement of matching perturbative QCD at high momentum transfer, guarantee the multiplicative renormalisability of the answer. Technically, the matching with perturbation theory is accomplished by the introduction of a transverse part to the quark-gluon vertex. We show that this transverse vertex is crucial for chiral symmetry breaking, and that massless solutions exist below a critical value of the strong coupling constant. Using the gluon propagator that we previously calculated, we obtain small corrections to the quark propagator, which keeps a pole at the origin in the chiral phase.Comment: 21 pages, 6 figures; McGill/94-24, SHEP 93/94-26 We generalise our results by showing that they are not sensitive to the specific choice that we make for the transverse vertex. We illustrate that fact in two new figure

Perturbative Evolution and Regge Behaviour

The known analytic properties of the Compton amplitude at small $Q^2$ place significant constraints on its behaviour at large $Q^2$. This calls for a re-evaluation of the role of perturbative evolution in past fits to data.Comment: Revised figures 4 and

GPDs of the nucleons and elastic scattering at high energies

Taking into account the electromagnetic and gravitational form factors, calculated from a new set of $t$-dependent GPDs, a new model is built. The real part of the hadronic amplitude is determined only through complex $s$. In the framework of this model the quantitative description of all existing experimental data at $52.8 \leq \sqrt{s} \leq 1960 \$GeV, including the Coulomb range and large momentum transfers ($0.0008 \leq |t| \leq 9.75 \ $GeV$^2$), is obtained with only$3$fitting high energy parameters. The comparison with the preliminary data of the TOTEM Collaboration at an energy of$7$ TeV is made.Comment: 14 pages, 21 figures, minor corrections (slightly brush English and removed two misprint in numbering

Consequences of t-channel unitarity for gamma(*)p and gamma(*) gamma(*) amplitudes

We show that t-channel unitarity constraints make it possible to obtain the photon-photon cross sections from the photon-proton and proton-proton cross sections, for Q2<150 GeV2. In order to do so, one must postulate the existence of double-pole or triple-pole singularities in the complex j plane.Comment: 3 pages, 3 figures, to appear in the proceedings of the 31st International Conference on High Energy Physics (ICHEP 2002), Amsterdam, The Netherlands, 24-31 July 2002, needs espcrc2.st