Thierry Gousset

Testing the handbag contribution to exclusive virtual Compton scattering

Abstract 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 + γ . 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.

Radio-detection signature of high-energy cosmic rays by the CODALEMA experiment

Taking advantage of recent technical progress which has overcome some of the difficulties encountered in the 1960’s in the radio detection of extensive air showers induced by ultra high energy cosmic rays (UHECR), a new experimental apparatus (CODALEMA) has been built and operated. We will present the characteristics of this device and the analysis techniques that have been developed for observing electrical transients associated with cosmic rays. We find a collection of events for which both time and arrival direction coincidences between particle and radio signals are observed. The counting rate corresponds to shower energies ≥ 5 × 10 16 eV. The performance level which has been reached considerably enlarges the perspectives for studying UHECR events using radio detection.

Geomagnetic origin of the radio emission from cosmic ray induced air showers observed by CODALEMA

The new setup of the CODALEMA experiment installed at the Radio Observatory in Nancay, France, is described. It includes broadband active dipole antennas and an extended and upgraded particle detector array. The latter gives access to the air shower energy, allowing us to compute the efficiency of the radio array as a function of energy. We also observe a large asymmetry in counting rates between showers coming from the North and the South in spite of the symmetry of the detector. The observed asymmetry can be interpreted as a signature of the geomagnetic origin of the air shower radio emission. A simple linear dependence of the electric field with respect to ∧ is used which reproduces the angular dependencies of the number of radio events and their electric polarity.

Exclusive production of pion pairs in {gamma}*{gamma} collisions at Large Q{sup 2}

The authors perform a QCD analysis of the exclusive production of two mesons in {gamma}*{gamma} collisions in the kinematical domain of large photon virtuality Q and small hadronic invariant mass W. This reaction is dominated by a scale invariant mechanism which factorizes into a perturbative subprocess, {gamma}*{gamma} {r{underscore}arrow} q{anti q} or {gamma}*{gamma} {r{underscore}arrow} gg, and a generalized two-meson distribution amplitude. The authors develop in detail the phenomenology of this process at e{sup +}e{sup {minus}} colliders. Using a simple model for the two-pion distribution amplitude, based on its general properties, the authors estimate the cross section for the kinematics accessible at BABAR, BELLE, CLEO and LEP.

Radioelectric Field Features of Extensive Air Showers Observed with CODALEMA

Based on a new approach to the detection of radio transients associated with extensive air showers induced by ultra high energy cosmic rays, the experimental apparatus CODALEMA is in operation, measuring about 1 event per day corresponding to an energy threshold ~ 5. 10^16 eV. Its performance makes possible for the first time the study of radio-signal features on an event-by-event basis. The sampling of the magnitude of the electric field along a 600 meters axis is analyzed. It shows that the electric field lateral spread is around 250 m (FWHM). The possibility to determine with radio both arrival directions and shower core positions is discussed.

Time ordering in off-diagonal parton distributions

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.

Heavy quarkonium hadron cross-section in QCD at leading twist

We compute the total cross section of a heavy quarkonium on a hadron target in leading twist QCD, including target mass corrections. Our method relies on the analytical continuation of the operator product expansion of the scattering amplitude, obtained long ago by Bhanot and Peskin. The cross section has a simple partonic form, which allows us to investigate the phenomenology of J/psi and Upsilon dissociation by both pions and protons.

Retardation Effect for Collisional Energy Loss of Hard Partons Produced in a QGP

We study the collisional energy loss suffered by an energetic parton travelling the distance L in a high temperature quark-gluon plasma and initially produced in the medium. We find that the medium-induced collisional loss ??E(L) is strongly suppressed compared to previous estimates which assumed the collisional energy loss rate ?dE/dx to be constant. The large L linear asymptotic behaviour of ??E(L) sets in only after a quite large retardation time. The suppression of ??E(L) is partly due to the fact that gluon bremsstrahlung arising from the initial acceleration of the energetic parton is reduced in the medium compared to vacuum. The latter radiation spectrum is sensitive to the plasmon modes of the quark-gluon plasma and has a rich angular structure.

Measuring gluon shadowing with prompt photons at RHIC and LHC

Abstract The possibility to observe the nuclear modification of the gluon distribution at small-x (gluon shadowing) using high- p ⊥ prompt photon production at RHIC and at LHC is discussed. The per-nucleon ratio, σ ( p + A → γ + X ) / ( A × σ ( p + p → γ + X ) ) , is computed for both inclusive and isolated prompt photons in perturbative QCD at NLO using different parameterizations of nuclear parton densities, in order to assess the visibility of the shadowing signal. The production of isolated photons turns out to be a promising channel which allows for a reliable extraction of the gluon density, R G A , and the structure function, R F 2 A , in a nucleus over that in a proton. Moreover, the production ratio of prompt photons at forward-over-backward rapidity in p–A collisions provides an estimate of R G A (at small x) over R F 2 A (at large x), without the need of p – p reference data at the same energy.

Are vertical cosmic rays the most suitable to radio detection

Abstract The electric field induced by extensive air showers generated by high energy cosmic rays is considered and, more specifically, its dependence on the shower incident angle. It is shown that for distances between the shower axis and the observation point larger than a few hundred meters, non-vertical showers produce larger fields than vertical ones. This may open up new prospects since, to some extent, the consideration of non-vertical showers modifies the scope of the radio-detection domain.