Dispersive approach in Sudakov resummation

The dispersive approach to power corrections is given a precise implementation, valid beyond single gluon exchange, in the framework of Sudakov resummation for deep inelastic scattering and the Drell-Yan process. It is shown that the assumption of infrared finite Sudakov effective couplings implies the universality of the corresponding infrared fixed points. This property is closely tied to the universality of the virtual contributions to space-like and time-like processes, encapsulated in the second logarithmic derivative of the quark form factor.Comment: 3 pages, talk given at Quark Confinement and the Hadron Spectrum 7, Ponta Delgada, Azores, Portugal, 2-7 Sep 2006, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included

Conformal window and Landau singularities

A physical characterization of Landau singularities is emphasized, which should trace the lower boundary N_f^* of the conformal window in QCD and supersymmetric QCD. A natural way to disentangle ``perturbative'' from ``non-perturbative'' contributions below N_f^* is suggested. Assuming an infrared fixed point is present in the perturbative part of the QCD coupling even in some range below N_f^* leads to the condition gamma(N_f^*)=1, where gamma is the critical exponent. This result is incompatible with the existence of an analogue of Seiberg free dual magnetic phase in QCD. Using the Banks-Zaks expansion, one gets 4<N_f^*<6. The low value of N_f^* gives some justification to the infrared finite coupling approach to power corrections, and suggests a way to compute their normalization from perturbative input. If the perturbative series are still asymptotic in the negative coupling region, the presence of a negative ultraviolet fixed point is required both in QCD and in supersymmetric QCD to preserve causality within the conformal window. Some evidence for such a fixed point in QCD is provided through a modified Banks-Zaks expansion. Conformal window amplitudes, which contain power contributions, are shown to remain generically finite in the N_f=-\infty one-loop limit in simple models with infrared finite perturbative coupling.Comment: 35 pages, 1 figure, JHEP style. A new section added to point out the results give some justification to the infrared finite coupling approach to power corrections, and suggest a way to compute their normalization from perturbative inpu

In search of foreign influences, other than French, in nineteenth-century Belgian court decisions

In many senses, nineteenth-century Belgium can be considered to be a 'legal province' of Belgium. In the tradition of the exegetical school, legislation is the one and only formal source of law for judicial decisions. This legislation is primarily composed of the Napoleonic codes. Judges seem to be afraid of referring to other sources. If a ‘foreign’ source is quoted, it is a French one

From stars to nuclei

We recall the basic physical principles governing the evolution of stars with some emphasis on the role played by the nuclear reactions. We argue that in general it is not possible from observations of stars to deduce constraints on the nuclear reaction rates. This is the reason why precise measurements of nuclear reaction rates are a necessity in order to make progresses in stellar physics, nucleosynthesis and chemical evolution of galaxies. There are however some stars which provides useful constraint on nuclear processes. The Wolf-Rayet stars of the WN type present at their surface CNO equilibrium patterns. There is also the particular case of the abundance of $^{22}$Ne at the surface of WC stars. The abundance of this element is a measure of the initial CNO content. Very interestingly, recent determinations of its abundance at the surface of WC stars tend to confirm that massive stars in the solar neighborhood have initial metallicities in agreement with the Asplund et al. (2005) solar abundances.Comment: 8 pages, 2 figures, be published in "European Physical Journal: Special Topics

Evidence for infrared finite coupling in Sudakov resummation

New arguments are presented in favor of the infrared finite coupling approach to power corrections in the context of Sudakov resummation. The more regular infrared behavior of some peculiar combinations of Sudakov anomalous dimensions, free of Landau singularities at large Nf, is pointed out. A general conflict between the infrared finite coupling and infrared renormalon approaches to power corrections is explained, and a possible resolution is proposed, which makes use of the arbitrariness of the choice of constant terms in the Sudakov exponent. A simple ansatz for a `universal' non-perturbative Sudakov effective coupling at large Nf emerges naturally from these considerations. An alternative evidence for an infrared finite {\em perturbative} effective coupling in the Drell-Yan process at large Nf (albeit at odds with the infrared renormalon argument) is found within the framework of Sudakov resummation for eikonal cross sections of Laenen, Sterman and Vogelsang.Comment: 5 pages; version 2: one sentence and one equation added (to appear in the Proceedings of the FRIF Workshop on First Principles Non-Perturbative QCD of Hadron Jets (Paris, 12-14 January 2006); version 3: additional material, comments and references (to appear in Physical Review D (Rapid Communication)