Probing gluon shadowing with forward photons at RHIC

There is a major need to better constrain nuclear parton densities in order to provide reliable perturbative QCD predictions at the LHC as well as to probe possible non-linear evolution at small values of x. In these proceedings, we discuss how the production of prompt photons at large rapidity in p-p and d-Au collisions at RHIC (sqrt(s_NN)=200 GeV) is sensitive to the nuclear modifications of gluon distributions at x~0.001 and at rather low scales, Q^2~10 GeV^2. The nuclear production ratio, R_dAu=sigma(d+A->gamma+X)/(2A sigma(p+p->gamma X), is computed for isolated prompt photons at NLO using the nDSg nuclear parton densities, in order to assess the visibility of the signal. We also emphasise that the expected counting rates in a year of running at RHIC are large, indicating that R_dAu could be measured with a high statistical accuracy.Comment: 4 pages. Talk given at Quark Matter 2008, Jaipur (India), February 4-10, 2008. Figure 2 replace

Les gerbes atmosphériques et leur contrepartie radio

École thématiqueThe main features of high-energy cosmic-ray air showers are explained before the question of their radio emissions is addressed. Les principales caractéristiques des gerbes atmosphériques issues des rayons cosmiques de grande énergie sont expliquées afin d'aborder le problème de leur émission radioélectrique

gamma* gamma -> pi pi at large Q^2

The QCD analysis of the process gamma* gamma -> pi pi at large Q^2 and small center-of-mass energy allows one to access a new hadronic observable describing the exclusive transition from a q qbar or g g state to a pair of mesons. A fruitful study may be envisaged at existing machines.Comment: 7 pages, to appear in the Proceedings of PHOTON 2000, Ambleside, England, August 200

Gluon radiation by heavy quarks at intermediate energies

Employing scalar QCD we study the gluon emission of heavy quarks created by the interaction with light quarks considered as dynamical scattering centers. We develop approximation formulas for the high energy limit and study when the full calculation reaches this high energy limit. For zero quark masses and in the high energy limit our model reproduces the Gunion-Bertsch results. We justify why scalar QCD represents a good approximation to the full QCD approach for the energy loss of heavy quarks. In the regime of accessible phenomenology we observe that the emission at small transverse momentum (dead cone effect) is less suppressed than originally suggested. We also investigate the influence of a finite gluon mass on the discussed results

Prompt Photon Production in p-A Collisions at LHC and the Extraction of Gluon Shadowing

International audienceA report is given on the study of using prompt photon production at the LHC to probe the gluon nuclear density, and more specifically the shadowing ratio GA/Gp that one could access in foreseen p-A runs

Identifying Prognostic Indicators for Electrical Treeing in Solid Insulation through PD Analysis

This paper presents early results from an experimental study of electrical treeing on commercially available pre-formed silicone samples. A needle-plane test arrangement was set up using hypodermic needles. Partial discharge (PD) data was captured using both the IEC 60270 electrical method and radio frequency (RF) sensors, and visual observations are made using a digital microscope. Features of the PD plot that corresponded to electrical tree growth were assessed, evaluating the similarities and differences of both PD measurement techniques. Three univariate phase distributions were extracted from the partial discharge phase-resolved (PRPD) plot and the first four statistical moments were determined. The implications for automated lifetime prediction of insulation samples due to electrical tree development are discussed

Radiative energy loss in the absorptive QGP: taming the long formation lengths in coherent emission

In an absorptive plasma, damping of radiation mechanisms can influence the bremsstrahlung formation in case of large radiation formation lengths. We study qualitatively the influence of this effect on the gluon bremsstrahlung spectrum off heavy quarks in the quark-gluon plasma. Independent of the heavy-quark mass, the spectrum is found to be strongly suppressed in an intermediate gluon energy region which grows with increasing gluon damping rate and increasing energy of the heavy quark. Thus, just as polarization effects in the plasma render the bremsstrahlung spectra independent of the quark mass in the soft gluon regime, damping effects tend to have a similar impact for larger gluon energies.Comment: contribution to the International Meeting "Excited QCD", Peniche, Portugal, 06 - 12 May 201

EAS radio detection at large impact parameter: the inverse problem and the design of a giant array

Contribution to the 30th ICRC, July 2007, Merida, MexicoInternational audienceExtensive air shower radio electric fields can be evaluated at large impact parameter with analytical expressions. Such a theoretical tool is most valuable in the present phase where the capabilities of the radio detection of extensive air shower are under investigations. It can help shaping strategies for the analysis of radio detection data. It can also be used to perform non trivial test of much more detailed numerical approaches which are currently under development. The approximation leading to such a formulation will be presented and two applications will be discussed: the "inverse" problem of how to go from a sampling of the radio electric field on a few antennas to the main characteristics of the extensive air shower, and the question of the antenna spacing of a giant array for ultra high energy cosmic rays

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.}Comment: 15 pages with 3 figure