Particle Physics / HighEnergy PhysicsSmashing 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 nuclearforce field), photons (waves in the electromagnetic field), leptons (including electrons and neutrinos), W and Z bosons (waves in the weak nuclearforce field, responsible for radioactivity), and Higgs bosons (the recent discovery of which provided smokinggun 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 matterantimatter asymmetry problem), earlyuniverse inflation, electroweak symmetry breaking and the hierarchy problem, quark confinement, and infiniteorder 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 Researchin fairly plain English "Whenever you're called on to make up your mind,
No  not so that chance shall decide the affair, (Piet Hein, Danish poet and friend of Niels Bohr)  
Most of my work concerns the physical modeling of highenergy 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 produces simulated particle 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 highperformance 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 realworld 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 spacetime 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.

What I really doin less fair EnglishEvent generators can be used to generate highenergyphysics "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 QCDbased models. The calculations are based on quantum field theory, spanning both rigorous solutions and phenomenological models, and are cast as Markov Chains, solved using randomnumber Monte Carlo methods. The physics areas include hard subprocesses, initial and finalstate 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 generalpurpose 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+1dimensional worldsheets with a tension of about 1 GeV/femtometre, and the stringbreakup process by quantum tunneling. My contributions include the development of transversemomentumordered 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 Yshaped "string junctions" which can appear in exotic newphysics decays and baryon beamremnants, models of colour reconnections in protonproton collisions and top quark decays, and interfaces to supersymmetric and other newphysics 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. The more specialized program VINCIA (a plugin to PYTHIA) explores a new method for generating perturbative corrections to highenergy 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 jetswithinjets and fluctuationswithinfluctuations. VINCIA works by first building up this semiclassical selfsimilar ("fractal") radiation pattern, using a formalism called leadingcolour 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 nonfractal (processdependent) quantum corrections are then finally imprinted on top of this "renormalizationgroupimproved" pattern, via an innovative approach unique to VINCIA: interleaved matrixelement 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 stateoftheart techniques, which are mainly based on separating phase space into "hard" (highenergy) and "soft" (lowenergy) 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.
