Jean-René Cudell

Benchmarks for the forward observables at RHIC, the Tevatron-run II, and the LHC

We present predictions on the total cross sections and on the rho parameter for present and future pp and (-)pp colliders, and on total cross sections for gammap-->hadrons at cosmic-ray energies and for gammagamma-->hadrons up to sqrt[s]=1 TeV. These predictions are based on an extensive study of possible analytic parametrizations invoking the largest hadronic dataset available at t=0. The uncertainties on total cross sections reach 1.9% at the Relativistic Heavy Ion Collider, 3.1% at the Tevatron, and 4.8% at the Large Hadron Collider, whereas those on the rho parameter are, respectively, 5.4%, 5.2%, and 5.4%.

Hadronic scattering amplitudes: Medium-energy constraints on asymptotic behavior

We consider several classes of analytic parametrisations of hadronic scattering amplitudes, and compare their predictions to all available forward data (p p, pbar p, pi p, K p, gamma p, gamma gamma, Sigma p). Although these parametrisations are very close for sqrt(s) > 9 GeV, it turns out that they differ markedly at low energy, where a universal pomeron term ~log^2(s) enables one to extend the fit down to sqrt(s)=4 GeV.

A QCD-inspired model for exclusive Vector meson production in deep inelastic scattering

Abstract Most of the features of the exclusive γ ∗ p →ϱ 0 p signal can be reproduced by a simple model which extends perturbative QCD to small momentum transfers in a natural way while linking pomeron behaviour with the gluon condensate. The data, however, seem to exhibit features characteristic of inelastic distributions, even after subtraction of a possible inelastic background.

Bounds on the Soft Pomeron Intercept

The Donnachie-Landshoff fit of total cross sections has now become a standard reference for models of total, elastic and diffractive cross-sections. Adopting their philosophy that simple-pole exchanges should account for all data to present energies, we assess the uncertainties on their fits, and show that the pomeron intercept which controls total cross sections and the real part of the elastic amplitude, can lie anywhere between 1.07 and 1.1, with a preferred value 1.094. supported in part by the USDOE contract DE-FG02-91ER 40688-Task A. supported in part by the Korea Science and Engineering Foundation through the Center for Theoretical Physics, Seoul National University, and by the Ministry of Education through contract BSRI-95-2427.Abstract The Donnachie-Landshoff fit of total cross sections has now become a standard reference for models of total, elastic and diffractive cross-sections. Adopting their philosophy that simple-pole exchanges should account for all data to present energies, we assess the uncertainties on their fits. Our best estimate for the pomeron intercept is 1.096−0.009+0.012, but several models have a good χ2 for intercepts in the range [1.07,1.11].

Particle production in a hadron collider rapidity gap: The Higgs case

Production of rare particles within rapidity gaps has been proposed as a background-free signal for the detection of new physics at hadron colliders. No complete formalism accounts for such processes yet. We study a simple lowest-order QCD model for their description. Concentrating on Higgs production, we show that the calculation of the cross section pp → ppH can be embedded into existing models which successfully account for diffractive data. We extend those models to take into account single- and double-diffractive cross sections pp → HX1X2 with a gap between the fragments X1 and X2. Using conservative scenarios, we evaluate the uncertainties in our calculation, and study the dependence of the cross section on the gap width. We predict that Higgs production within a gap of 4 units of rapidity is about 0.3 pb for a 100 GeV Higgs at the Tevatron, and almost 2 pb for a 400 GeV Higgs within a gap of 6 units at the LHC with √s = 14 TeV.

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.

Pseudoscalar vertex, Goldstone boson and quark masses on the lattice

Abstract We analyse the Structure Function collaboration data on the quark pseudoscalar vertex and extract the Goldstone boson pole contribution, in 1/p2. The strength of the pole is found to be quite large at presently accessible scales. We draw the important consequences of this finding for the various definitions of quark masses (short distance and Georgi-Politzer), and point out problems with the operator product expansion and with the non-perturbative renormalisation method.

Quark and gluon condensates and the small x limit of the nucleon structure functions

Abstract We calculate the non-perturbative contributions from the quark and gluon condensates to the nucleon structure functions at small x in the asymptotic region. We show that for values of Q 2 around 10 GeV 2 , these contributions dominate the perturbative ones and agree reasonably with present data.

Theory and phenomenology of the gluon propagator from the Dyson-Schwinger equation in QCD

Abstract A new solution is found for the Dyson-Schwinger equation for the gluon propagator in the axial gauge. Unlike previously found ones this propagator does not display a 1 k 4 pole as k → 0 but has a very soft singularity in this region. Thus it represents a gluon which is not confining but rather confined since in configuration space the propagator vanishes for large impact parameters. This solution is relevant for processes in which soft gluons are exchanged, such as physics at low x or equivalently the construction of the QCD pomeron. Some of the phenomenological consequences are discussed.

Axions and polarisation of quasars

We present results showing that, thanks to axion‐photon mixing in external magnetic fields, it is actually possible to produce an effect similar to the one needed to explain the large‐scale coherent orientations of quasar polarisation vectors in visible light that have been observed in some regions of the sky.