Angularities and other shapes

I discuss soft-gluon resummation and power corrections for event shape distributions, mostly in e+ e- annihilation. I consider specifically the thrust, the C parameter, and the class of angularities, and show how factorization techniques and dressed gluon exponentiation lead to predictive models of power corrections that are firmly grounded in perturbative QCD. The scaling rule for the shape function for angularities is derived as an example. Finally, I make a few remarks on possible generalizations to hadron collisions, and on their relevance to LHC studies.Comment: 10 pages, contribution to the FRIF workshop on first principles non-perturbative QCD of hadron jets, LPTHE, Paris, France, 12-14 Jan 200

The growing toolbox of perturbative QCD

Advances in perturbative QCD techniques have been crucial for the successful interpretation of the data collected in Run I of LHC, and for the discovery of the Higgs boson. I will very briefly highlight some recent additions to the QCD toolbox, and note how these new tools are likely to be essential for future precision physics, both in Run II at the LHC, and in view of future hadron and lepton colliders.Comment: Talk given at the Workshop "LFC15: Physics Prospects for Linear and Other Future Colliders", ECT*, Trento, 7-11 September 201

Analytic resummation and power corrections for DIS and Drell--Yan

Dimensional continuation is applied to resummed expressions for the DIS and Drell-Yan partonic cross sections, to regularize the Landau pole. Simple analytic expression are obtained, encoding information about nonperturbative power--suppressed effects.Comment: 6 pages, talk delivered at DIS 2001, Bologna (Italy), 27/4-1/5 200

On power corrections to event shapes

Recent work on the theme of power corrections in perturbative QCD is briefly reviewed, with an emphasis on event shapes in e+ e- annihilation. The factorization of soft gluon effects is the main tool: it leads to resummation, and thus highlights the limitations of perturbation theory, pointing to nonperturbative corrections whose size can be estimated. Power corrections can be resummed into shape functions, for which QCD--based models are available. Theoretical progress is closing in on the nonperturbative frontier.Comment: 15 pages, no figures. Invited talk at the XIV Italian Meeting on High Energy Physics, Parma, April 2002. Extended versio

Progress on the infrared structure of multi-particle gauge theory amplitudes

I will review some of the recent intense activity concerning infrared and collinear divergences in gauge theory amplitudes. The central quantity in these studies is the multi-particle soft anomalous dimension matrix, which is completely known at two loops for both massless and massive particles, and whose properties are currently being studied at three-loops and beyond. I will describe how, in the massless case, the simple dipole-like structure of the anomalous dimension up to two loops can be exploited in the high-energy limit to study effects that go beyond the standard form of Regge factorization. Furthermore, I will briefly review some of the techniques that have recently been developed to compute the soft anomalous dimension at high orders in perturbation theory, and I will give some examples of applications, including a result valid to all orders in perturbation theory for a specific class of diagrams.Comment: 13 pages, 1 figure. Contribution to the Proceedings of "Loops and Legs in Quantum Field Theory", 27 April - 2 May 2014, Weimar, German

Sudakov logs and power corrections for selected event shapes

I summarize the results of recent studies analyzing perturbative and nonperturbative effects of soft gluon radiation on the distributions of the C-parameter and of the class of angularities, by means of dressed gluon exponentiation.Comment: Talk given at DIS 2004, Strbske Pleso, Slovakia, 14-18 April 200

String derivation of two-loop Feynman diagrams

We briefly review the technology involved in extracting the field-theory limit of multiloop bosonic string amplitudes, and we apply it to the evaluation of simple two-loop diagrams involving scalars and gauge bosons.Comment: 7 pages, 1 figure, contributed to Beyond the Standard Model V, Balholm, Norway, April 1997. Latex with aipproc.st

Infrared singularities in QCD amplitudes

We review recent progress in determining the infrared singularity structure of on-shell scattering amplitudes in massless gauge theories. We present a simple ansatz where soft singularities of any scattering amplitude of massless partons, to any loop order, are written as a sum over colour dipoles, governed by the cusp anomalous dimension. We explain how this formula was obtained, as the simplest solution to a newly-derived set of equations constraining the singularity structure to all orders. We emphasize the physical ideas underlying this derivation: the factorization of soft and collinear modes, the special properties of soft gluon interactions, and the notion of the cusp anomaly. Finally, we briefly discuss potential multi-loop contributions going beyond the sum-over-dipoles formula, which cannot be excluded at present.Comment: 20 pages, 3 figures. To be published in the proceedings of 23rd Les Rencontres de Physique de la Vallee d'Aoste, La Thuile, Aosta Valley, Italy, 1 - 7 March 200

Impact of large-x resummation on parton distribution functions

We investigate the effect of large-x resummation on parton distributions by performing a fit of Deep Inelastic Scattering data from the NuTeV, BCDMS and NMC collaborations, using NLO and NLL soft-resummed coefficient functions. Our results show that soft resummation has a visible impact on quark densities at large x. Resummed parton fits would therefore be needed whenever high precision is required for cross sections evaluated near partonic threshold.Comment: 5 pages, 5 figures. Talk given at DIS 2005, XIII Workshop on Deep Inelastic Scattering, April 27-May 1, 2005, Madison, WI, U.S.