Particle Physics / High-Energy Physics

Smashing Atoms Since 1932

Combining relativity and quantum mechanics, modern particle physics explores the subatomic world of quarks (building blocks of nuclear matter), gluons (waves in the strong nuclear-force field), photons (waves in the electromagnetic field), leptons (including electrons and neutrinos), W and Z bosons (waves in the weak nuclear-force field, responsible for radioactivity), and Higgs bosons (the recent discovery of which provided smoking-gun evidence for the existence of the Higgs field, through which we now know elementary particles acquire mass).

Those are the only fundamental quantum fields (a.k.a. elementary particles) we know of. But not everything about them is well understood, nor do we know if there are more such fields in nature. In fact, mysteries abound, and are discussed in technical lingo under names like dark matter and dark energy, baryogenesis (the matter-antimatter asymmetry problem), early-universe inflation, electroweak symmetry breaking and the hierarchy problem, quark confinement, and infinite-order resummations, to name but a few. Also, we can't hide from gravity forever.

To find out more, we use some of the largest machines on Earth - particle accelerators. The Large Hadron Collider at CERN, near Geneva, Switzerland, was recently upgraded to reach energy scales of 13 TeV, sufficient to resolve distance scales 10,000 times smaller than a proton. The challenge for theorists is to try to think thoughts - and do calculations - that match.

My Research

in fairly plain English

"Whenever you're called on to make up your mind,
and you're hampered by not having any,
the best way to solve the dilemma, you'll find,
is simply by spinning a penny.

No - not so that chance shall decide the affair,
while you're passively standing there moping;
but the moment the penny is up in the air,
you suddenly know what you're hoping"

(Piet Hein, Danish poet and friend of Niels Bohr)

Most of my work concerns the physical modeling of high-energy particle collisions. The solutions are cast in the form of computer programs called "Monte Carlo event generators". Think of an event generator as a "Virtual Collider" which spits out simulated collisions much like those that are produced in the real experiments. The simulations can run on anything from a laptop (for simple calculations) to supercomputer clusters, and high-performance computing is an important aspect of our research. The name "Monte Carlo" owes to the calculations being based on random numbers and probabilities, reminiscent of games of chance, with the rules in our case dictated by quantum physics.

By comparing simulated particle collisions ("theory") to real-world measurements ("experiment"), we test the theoretical models, whether they agree with nature - or not. Discrepancies are always interesting: they indicate that there is something to learn. Often, the lesson is simply that the range of validity of an approximation made in the modelling has to be extended (typically not a simple thing to do or someone else would already have done it, so these cases provide plenty of work to keep us busy); occasionally however, it can be the first sign of a genuinely new phenomenon - the "jackpot" case.

Within the context of the simulations, we can play with the physical laws: introduce new forces or new types of fundamental particles - even change the number of space-time dimensions. Thus, we can gain insight into how new laws of nature would manifest themselves in experiments. This is not only interesting from a theoretical point of view, but has important practical applications. Our experimental colleagues make extensive use of our simulations to plan and design the experiments, and to optimize their searches for "new physics".

We can also modify the way we model or approximate the known laws of nature. By varying the model assumptions we can estimate the resiliency of the theoretical predictions, identifying areas where improvements are needed or where additional physical phenomena may play a role, avoiding conclusions drawn on uncertain grounds. And we can explore new approaches to solving or modeling the underlying physical theory. This is where most of my work goes, trying to achieve as high a level of detail and accuracy as possible.

My speciality is the strong nuclear force, encapsulated by the theory of quantum chromodynamics (QCD), which governs the dynamics of quarks and gluons. This is arguably the most complicated of nature's elementary forces, and it exhibits some spectacular emergent phenoma which are still only partly understood; such as "jets" (collimated sprays of nuclear radiation) and "strings" (not superstrings, these kinds of strings are also called vortex lines). Delving into these phenomena - and building new state-of-the art dynamical models for them - is a fascinating and enjoyable field of research. Incidentally, QCD strings are capable of exerting a pull sufficient to lift a 16-ton truck, quite a "strong force" indeed when applied to a single elementary particle. Its extreme strength is in fact is one of the main challenges in QCD, which render many problems nigh impossible to solve mathematically.

The core of my research is concerned with physics at the Large Hadron Collider (proton-proton collisions), jets, jet substructure, string fragmentation (the formation and breakup of QCD strings), and the interplay between fixed- and infinite-order approximations to quantum field theory. Since event generators naturally span a wide range of phenomena, I frequently deal with forces other than QCD as well, both known and hypothetical. Indeed, the fact that this research spans a huge range of energies and phenomena is one of the aspects I find attractive about it, as well as jets and strings really being quite beautiful things, once you get to know them.

I also aim to get the public involved in high energy research, for instance via LHC@home 2.0, a volunteer computing project developed at CERN, online since 2010, and for which I was the lead scientist on the first pilot project, called Test4Theory. Participants can download their own virtual colliders and run them on their home computers (eg at times when they would otherwise just be "sleeping"), contributing valuable computing power to theory calculations for the LHC. Incidentally, this was the first volunteer computing project in the world to use virtualization technology, an aspect developed specifically for this project.

What I really do

in less fair English

Event generators can be used to generate high-energy-physics "events", i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, within and beyond the Standard Model of particle physics, with emphasis on those where strong interactions play a role, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description: instead the programs have to be based on a combination of analytical results and various QCD-based models.

The calculations are based on quantum field theory, spanning both rigorous solutions and phenomenological models, and are cast as Markov Chains, solved using random-number Monte Carlo methods. The physics areas include hard subprocesses, initial- and final-state parton showers, underlying events and beam remnants, fragmentation and decays, and much more. The programming language of choice is currently C++, with predecessors in FORTRAN.


The event generator PYTHIA is a general-purpose vessel for exploring phenomena both within and beyond the Standard Model. It is one of the most widely used tools in high energy physics. Its hallmark feature is the "Lund string model", used to model "hadronization" (the process by which quarks and gluons turn into hadrons). Strings are a universal phenomen that appears in extremely diverse physical systems, from superconductors and superfluids to cosmic strings. The strings in QCD enforce "confinement" - their strong pull locking quarks and gluons away inside nucleons, unable to escape. (If this sounds slightly similar to what black holes do by way of gravity, then perhaps there are even things to be learned by comparing the physics of strings to that of black holes. That's one current research question I would love to know the answer to.) At present, we model QCD strings as simple 1+1-dimensional worldsheets with a tension of about 1 GeV/femtometre, and the string-breakup process by quantum tunneling. My contributions include the development of transverse-momentum-ordered parton showers, a more sophisticated model of the "underlying event" which forms the basis of much of the current modelling of LHC collisions, a fragmentation model for Y-shaped "string junctions" which can appear in exotic new-physics decays and baryon beam-remnants, models of colour reconnections in proton-proton collisions and top quark decays, and interfaces to supersymmetric and other new-physics scenarios. By careful examination of the available experimental constraints on the physics models, I have also provided some of the main parameter sets ("tunes") used by both the experimental and phenomenological communities for theory comparisons, such as the "Perugia" family of tunes for PYTHIA 6 and the more recent "Monash" tune for PYTHIA 8.

V I N C I A The more specialized program VINCIA (a plug-in to PYTHIA) explores a new method for generating perturbative corrections to high-energy scattering processes, in particular those associated with "jets" in quantum chromodynamics (QCD). It is based on the observation that jets have fractal patterns inside them, quantum structures of jets-within-jets and fluctuations-within-fluctuations. VINCIA works by first building up this semi-classical self-similar ("fractal") radiation pattern, using a formalism called leading-colour antenna factorization (adapted for implementation as a Monte Carlo Markov Chain algorithm). By taking the running coupling into account, trivial departures from true scale invariance can be taken systematically into account. Genuinely non-fractal (process-dependent) quantum corrections are then finally imprinted on top of this "renormalization-group-improved" pattern, via an innovative approach unique to VINCIA: interleaved matrix-element corrections. The latter are calculated systematically order by order in perturbative quantum field theory, a task which today is mainly handled by other computer programs, rather than by hand. The approach to quantum corrections taken in VINCIA contrasts greatly with other state-of-the-art techniques, which are mainly based on separating phase space into "hard" (high-energy) and "soft" (low-energy) regions, and merging together many separate event samples, one for each order of perturbative corrections. The formalism we are developing for VINCIA allows a single unweighted event sample to be produced in one go, with very high sampling efficiencies and high CPU speeds. It also allows for a wide scope of automated uncertainty evaluations, giving it the ability to make a comprehensive estimate of how reliable it thinks its answers are, for each generated event.