Older talks

This page contains a list of talks given between late 1999 and early 2008. See also the list of recent talks.

Review talks

General QCD
QCD final-states
High-energy limit of QCD

Research talks

Jets, event shapes, resummations and power corrections
High-energy scattering and small-x

Review talks

General QCD

Recent developments in perturbative QCD (Invited plenary talk at DIS 2006, Tsukuba, Japan, April 2006)
A review of a topics in QCD that have been particularly active in the past couple of years, covering in particular twistor-based recursion techniques at tree level; twistor approaches and automatation of standard techniques for 1-loop calculations; the status of NNLO jet calculations; and discussion of non-trivial structures that appear in higher-order calculations.
Developments in perturbative QCD (Invited plenary talk at Lepton-Photon 2005, Uppsala, Sweden, 30 June - 5 July 2005)
A brief review of key developments and ongoing projects in perturbative QCD over the past 2 years.

QCD final-states

Opening talk for jets subgroup of SM handles and candles working group and Hopes for LHC jets from a theory perspective (both given in the context of the Physics at TeV Colliders workshop, Les Houches, June 2007).
These two talks are intended to provide an introduction to some of the current issues in jet physics that could be addressed by the Les Houches workshop, together with a set of minimal hopes for what might be a minimum set of requirements for LHC jet-finding.
Higher-orders, jets and the interplay between them (invited talk at the Paris CDF Collaboration Meeting, June 2007).
This talks takes a brief look at recent developments in higher-order QCD calculations for colliders (NLO, NLO with parton showers, NNLO) and then proceeds to discuss how higher orders are connected with jets, discussing infrared safety issues, seedless cone definitions, hadronisation and underlying event in the context of top mass measurements, and jet flavour.
Outils de pointe pour la reconstruction QCD aux collisionneurs (in French, talk at the Colloque ANR des Jeunes Chercheurs SDU Physique, Orléans (France), April 2007).
An overview current themes in jet physics and and overview of recent work carried out by the Jussieu group on the subject.
Overview of the FRIF workshop on first principles non-perturbative QCD of hadron jets (seminar given at the LPT, Orsay, January 2006).
Analytical approaches to hadronisation have been extensively tested in the context of jet-physics, where non-perturbative eects can be as large as the NLO perturbative corrections. There exists a vast body of data from the LEP and HERA colliders and a variety of theoretical approaches, many yet to be fully explored. The current situation is somewhat ambiguous: different analyses lead to different conclusions; in some cases the theory is probably incomplete; in others there may still be defficiencies in the experimental analyses. This situation needs to be resolved rapidly, while the LEP and HERA experimenters still remain active in the subject. And it's about time to start addressing similar issues at hadron colliders. The aim of this small workshop (about 30 experimenters and theorists) is to help put together an overall picture of the situation and to establish where further work could usefully be carried out, both experimentally and theoretically.
Hadronic final states and resummation (Plenary talk at opening meeting of HERA/LHC workshop, March 2004)
While some areas of the HERA programme, notably structure functions, have an obvious impact on LHC physics, for final-state measurements the connection is less immediate. This talk examines two possible areas of contact: the low-x, moderate-Q region where there is a need to understand whether the low-x effects that are systematically seen in many HERA measurements could be relevant at the LHC; and the moderate-x, high-Q region where HERA has acquired considerable expertise in extracting valuable information from the subtleties of the QCD final state.
QCD tests through hadronic final-state measurements (Opening plenary talk at DIS 2002).
The testing of QCD has evolved a long way since its origins over twenty years ago, and today goes far beyond the problem of establishing whether QCD is the correct theory of hadronic interactions. With the aid of a small selection of examples involving hadronic final states, this talk illustrates a modern (and personal) view of what is meant by testing QCD.

High-energy limit of QCD

Recent progress in QCD at high energies (Informal workshop on theoretical physics, Cortona (Italy), May 2004).
A brief introduction to the physics of the high-energy limit of QCD, including some recent developments, intended for a general theoretical physics audience.
High-energy QCD and the search for large perturbative field strengths (Heraeus workshop on QFT in Particle and Solid State Physics, June 2003).
An introduction to the physics and issues of high-energy QCD, intended for a general theoretical physics audience.
Small-x (a micro review) (Cambridge workshop on TeV-scale physics, July 2002).
A quick overview of small-x physics, including a discussion of experimental evidence for BFKL, the status of NLL corrections and some recent progress and considerations on saturation. Intended for a general (theoretical/experimental) QCD audience.
Interactions lepton-nucléon UHE en QCD (CAP6 meeting, Univ. Paris 6, March 2002; in French).
A brief review of some aspects of the status of our knowledge about high-energy lepton-nucleon scattering, as relevant to the detection and propagation of ultra-high-energy cosmic rays.

Research talks

Jets, event shapes, resummations and power corrections,

Jet algorithms (invited talk to the CMS JetMET working group, CERN, Switzerland, March 2008).
An introduction to the behaviour and properties of common jet algorithms. It discusses infrared and collinear safety issues in legacy algorithms, with mention of the logic behind the approaches we have taken to address these issues, and closes with an illustration of area-based pileup subtraction.
Theoretical aspects of jet-finding (invited talk at the 4th ATLAS hadronic calibration workshop, Tucson (AZ), USA, March 2008).
A lecture providing an overview of some of the main issues in jet-finding, recent progress and an illustration of an advanced jet-finding application in Higgs searches.
Jets, our window on partons at the LHC (seminar given in the Rutgers High Energy Theory group, November 2007, and at the LPT Orsay, January 2008)
Jets, collimated bunches of hadrons that result from the fragmentation of quarks and gluons, will be key observables at LHC. This talk introduces the issues that are relevant in hadron-collider jet-physics and reviews recent developments. These include both technical breakthroughs, finally enabling hadron-collider jet-finding to be brought up to the standards set out in the 1990 Snowmass accord, and progress in understanding the physics of jets, which serves as groundwork for optimising the practical effectiveness of jet-finding at LHC.
Recent progress in defining and understanding jets (talk given at the International Symposium on Multiparticle Dynamics, Berkeley, USA, August 2007).
This talk reviews some of the main developments that have occurred in jet finding in the past couple of years, including: 1) technical advances such as fast computational approaches to the kt and Cambridge/Aachen algorithms, and the complete formulation of a proper (seedless) cone algorithm; 2) progress in understanding quantitative behaviour of jet algorithms with respect to underlying event and hadronisation corrections; 3) work that exploits the better understanding so as to improve the results from jet algorithms.
A practical seedless infrared-safe cone algorithm (talk given at Rencontres de Moriond: QCD and Hadronic interactions, La Thuile (Italy), March 2007).
A brief introduction to the infrared safety issue in the midpoint (seeded) cone algorithm and an explanation of how to construct an efficient seedless and infrared-safe cone algorithm, together with an illustration of its impact for a small choice of physical observables.
Characterising non-perturbative effects in jets (talk given at Ringberg workshop on non-perturbative QCD of jets., Ringberg, January 2007).
This talk examines various non-perturbative contributions to jet momenta in a hadron-collider context. The main results are that hadronisation effects scale as 1/R, underlying event and pileup scale scale as R², with the same coefficient for all jet algorithms to first order. Some preliminary results are given also for higher-order effects.
Jet-finding for LHC (talk given at Robi is 60, SPhT Saclay, November 2006).
This talk gives an overview of some recent progress on jet-finding for the LHC environment. Topics addressed include: (a) the practical issue of speed, and the improvements that can be obtained by making use of techniques from computational geometry; and (b) the question of how jet algorithms behave in the presence of large contamination from pileup and how one can correct for this.
Jets in heavy ion collisions with fast clustering jet finders (talk given at Quark Matter 2006, Shanghai, November 2006).
Jets have never been observed so far in heavy ion collisions, in part due to the difficulty of extracting them from the huge background. We propose the use of a sequential recombination jet-finder (such as the kt and Aachen/Cambridge jet finders) for this task. Such jet-finders are favoured in QCD because of their ability to approximately invert the QCD branching sequence. Their use in high multiplicity environments has so far been hindered by issues of speed (they were thought to take a time proportional to the cube of the number of particles) and the background subtraction. This talk will discuss some progress which has been made on both these issues recently.
Recent developments in jet clustering algorithms (talk given in Florence, June 2006 and Wuppertal, October 2006).
Two broad classes of jet finders are in common use: iterative cone and sequential clustering (kt) algorithms. After illustrating why many theorists prefer clustering algorithms, this informal talk will discuss recent results and work in progress on adapting them to the LHC era. Issues addressed will include: their computational speed in high-multiplicity environments and how to better understand and correct for underlying event and minimum bias contamination.
Infrared safe definition of jet flavour (talk given at XLI Rencontres de Moriond (QCD and high energy hadronic interactions, La Thuile, Italy, March 2006, and at DIS 2006, Tsukuba, Japan, April 2006).
It is common, in both theoretical and experimental studies, to separately discuss quark and gluon jets. However, even at parton level, widely-used jet algorithms fail to provide an infrared safe way of making this distinction. This talk examines the origin of the problem, and proposes a solution in terms of a new "flavour-kt" algorithm. As well as being of conceptual interest this can be a powerful tool when combining fixed-order calculations with multi-jet resummations and parton showers. It also has applications to studies of heavy-quark jets.
Taking QCD beyond fixed order perturbation theory - systematically (seminar for general theory audience given at SPhT, Saclay (CEA), November 2005, Edinburgh, May 2006, Florence, June 2006, CERN, August 2006 and Zurich, December 2006).
The standard theoretical tool of perturbative QCD, fixed-order perturbation theory, often leads to series expansions in the coupling that are insufficiently convergent to be of direct practical use. This is especially true when making predictions for hadronic final states. Over the past 20 years methods have been developed for analytically calculating dominant contributions at all orders of perturbation theory. These "resummation" techniques have been applied with success to comparisons with a number of experimentally well-measured observables. The first part of this talk will introduce the ideas and structures that appear in resummed calculations, many of which are of remarkable simplicity. Despite this simplicity, actually carrying out resummations has in the past proved to be a delicate craft. Recently, significant progress has been made towards developing a systematic framework for final-state resummations, as will be described in the second part of this talk.
Automated resummation and hadron-collider event shapes (Radcor 2005, Internation symposium on radiative corrections, Shonan Village, Japan, October 2005).
This talk gives an introduction to the theoretical understanding that lies behind automated resummation. It then discusses how it is implemented in practice and illustrates its flexibility through applications to hadron-collider event shapes.
Event shapes for hadron colliders (HERA LHC workshop, CERN, Geneva, January 2005).
+ more extended version (LPNHE, Jussieu, Paris, France, July 2005, and MPPMU, Munich, July 2007).
Event shapes combine conceptual simplicity with sensitivity to QCD dynamics over a range of scales and have been the subject of innumerable studies in e⁺e^- and DIS collisions. This talk examines the new questions that can be addressed by extending event shapes to hadron colliders, and presents some concrete examples of how to design hadron-collider event shapes so as to ensure that they are both measurable experimentally and calculable theoretically. First results for resummed distributions are presented based on CAESAR.
CAESAR: Computer automated resummations (DIS 2004, Strbske Pleso, Slovakia, April 2004).
+ similar talk, with a few more explanations (9ème rencontre Itzykson, Paris, France, June 2004).
This talk discusses gives a brief introduction to the motivations and principles of computer auomated resummations for final-state observables. See also qcd-caesar.org.
Generalized resummation of final-state observables (Manchester University, November 2003).
All-orders perturbative resummation of dominant logarithmically enhanced contributions is an important prerequisite for many studies of final states, in particular when one places strong restrictions on the overall amount of final-state radiation. Up to now such resummations have been carried out by hand for a single observable and process at a time. After an introduction to the subject, this talk discusses a general procedure for resumming arbitrary observables. The method is designed such that a computer program can automatically carry out resummations given just a subroutine for an observable. As an application, the first next-to-leading logarithmic resummed predictions for hadronic-dijet event shapes are shown.
Non-global logarithms: an overview (Moriond QCD, March 2003).
So called non-global logarithms are terms (widely neglected until recently) that appear in a range of contexts, such as jet shapes, interjet energy flows and perturbative gap-survival probabilities. This talk examines the kinds of process where they appear, what is known about how to calculate them, and their practical impact on QCD predictions.
DIS event-shape resummations and spin-offs (Moriond QCD, March 2002 and DIS 2002, April-May 2002).
This talk presents results from a recently completed project to calculate NL resummed distributions for half a dozen different event shapes in the 1+1-jet limit of DIS. This allows fits for the strong coupling and of non-perturbative effects using the vast amounts of data (in existence and to come) on these observables from H1 and ZEUS. Spin-offs include: the discovery of a new class of logs for certain final state obserables (non-global logs); the development of state-of-the-art PDF evolution code; and a program that allows a speed-up by an order of magnitude of certain fixed-order calculations in DIS with DISENT or DISASTER++.
Stretching QCD perturbation theory into the non-perturbative limit (CERN theory colloquium, May 2001, also Saclay-SPhT and IPN/CPhT seminars).
At high orders, perturbation theory is known to break down, because it diverges factorially. In QCD this breakdown is in part associated with physics in the infrared and, in recent years, it has been suggested that it can be exploited to deduce information about non-perturbative corrections to a range of observables. This talk reviews these ideas and discuss how they fare in comparisons to experimental data.
Putting the hadrons into hadronisation (DESY and Cambridge, Spring 2001 and CPT-Marseille, February 2003).
In recent years people have started using `universal' 1/Q corrections to account for hadronisation effects in event shapes. Generally these contributions are calculated assuming massless particles. This talk looks at what happens when one takes hadron masses into account, and shows they are responsible for effects that are as large as the `normal' 1/Q corrections. We will examine how these effects depend on Q, and to what extent they can be eliminated by suitable redefinitions of the observables.

High-energy scattering and small x

Matrix combination of BFKL and DGLAP (talk given at the International Symposium on Multiparticle Dynamics, Berkeley, USA, August 2007).
Longer seminar version (given in the BNL theory group, December 2007).
A progress report on a long-term project to put together DGLAP and the linear regime of BFKL evolution, including higher order and running-coupling corrections, with emphasis on developments related to the incorporation of the full matrix (flavour) structure of the evolution.
Impact of higher orders in the high-energy limit of QCD (seminar at the HEP group, Cavendish laboratory, Univeristy of Cambridge, UK, March 2005).
+ similar talk with a few more explanations (seminar at BNL theory group, Brookhaven National Laboratory, NY, USA, May 2005 and at Fermilab theory group, Illinois, USA, May 2005)

Since the earliest days of QCD there have been investigations into the high-energy limit of the theory. The cornerstone prediction of the field was a spectacularly rapid rise of perturbative cross sections at high energies. This, however, is not borne out by the data. The past decade has seen extensive work on trying to understand this apparent contradiction, in particular via the calculation of higher order corrections. This talk will give an introduction to the origin of the rise of QCD cross sections, showing also the comparisons to data. We will then illustrate some difficulties that arise when including next-to-leading corrections and discuss the extent to which it is possible to make robust predictions including higher-order corrections.
Assorted NLL small-x comments (with emphasis on preasymptotics) (QCD at cosmic energies, Erice, Italy, August 2004).

One of the most striking features of renormalisation-goup-improved NLLx studies of the small-x gluon-gluon splitting function, is the considerable size of the preasymptotic effects. As a result, the splitting function starts to grow only for x<0.001. This apparently slow onset of resummation effects can be understood as a consequence both of NLLx and running-coupling effects. The potential impact on phenomenology is illustrated by convoluting the splitting function with a typical gluon distribution.
Fall and rise of the gluon splitting function (at small x) (DIS 2004, Strbske Pleso, Slovakia, April 2004).
+ similar talk, for more general audience (Eighth Workshop on Non-Perturbative QCD, Paris, France, June 2004).
+ a more extended version of the talk (HERA LHC workshop, CERN, Geneva, January 2005).
It is generally assumed that the LO, NLO, NNLO,... hierarchy for the gluon splitting function breaks down when alpha_s log 1/x sim 1. It turns out though that because of the particular structure of the LLx series, the breakdown actually occurs much earlier, for alpha_s log^2 1/x sim 1. This talk discusses the origin of this breakdown and puts it in the context of recent resummed calculations for the small-x gluon splitting function.
Hacking CCFM (Lund small-x workshop, February 2001).
The CCFM equation as it is usually defined contains the basic elements needed to obtain small-x final states and total cross section to leading logarithmic order. Recently other calculations at small x have implied that it is important to include certain collinearly-enhanced corrections, related to energy scales and the full splitting function. This talk discusses how one might go about including some of these effects in the CCFM equation.
Recent developments in small-x physics (CERN, January 2000).
This talk reviews some of the major developments that have taken place in small-x physics in the past couple of years: the completion of the epic (10-year long!) calculation of the NLO corrections; the discovery that their inclusion leads to negative cross sections; the solution to the problem, by inclusion of collinearly-enhanced higher order contributions; and the understanding at all orders of how the running of the coupling affects the evolution and anomalous dimensions.
Factorisation and diffusion at small x (Florence QCDNET meeting, September 1999).
This talk considers various BFKL-like collinear models in order to deduce information about factorisation and diffusion at small x. It includes a discussion of the nature of power corrections to factorisation. It also introduces the concept of tunneling, a phenomenon which could replace diffusion at very high energies.

Some associated information is to be found on the tunneling pages.