This page contains a list of talks given between late 1999 and early
2008. See also the list of recent talks.
Recent developments in perturbative QCD (Invited plenary talk at
DIS 2006, Tsukuba, Japan,
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
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.
- 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
- 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
- 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
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
- QCD tests through
hadronic final-state measurements (Opening plenary talk at DIS
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
High-energy limit of QCD
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
- 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
- Small-x (a
micro review) (Cambridge workshop on TeV-scale physics, July
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
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
Jets, event shapes, resummations and power
algorithms (invited talk to the
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.
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
- 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
- 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,
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
Event shapes for hadron colliders (HERA LHC workshop, CERN, Geneva,
+ 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.
resummation of final-state observables (Manchester University,
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.
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.
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
- Stretching QCD
perturbation theory into the non-perturbative limit (CERN
theory colloquium, May 2001, also Saclay-SPhT and IPN/CPhT
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
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).
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
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
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,
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
- 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
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
Some associated information is to be found on the tunneling pages.