PERT AND NON-PERT QCD
EW AND SYMMETRY BREAKING
BEYOND THE SM
HEAVY FLAVOUR PHYSICS
NEUTRINO AND NON ACCEL PHYSICS
Quantum Chromodynamics (QCD) is the theory of the strong interaction, one of the four fundamental forces in nature. It describes the interactions between quarks and gluons, and in particular how they bind together to form hadrons. One of the triumphs of modern particle physics has been the extent to which QCD has successfully accounted for the strong interaction processes observed at high energy particle colliders, and particularly phenomena such as `jet' production. Reliable and quantitative QCD calculations are nowadays essential in understanding the enormous amount of high quality experimental data available, particularly since many sources of new physics are obscured by standard QCD processes.
Alan Martin, James Stirling
The partonic (quark and gluon) structure of the proton are central to understanding all high-energy hard scattering processes initiated by protons. The `MRS' series of partons, a joint Durham-RAL collaboration, are widely used worldwide and are always amongst the most highly cited academic papers in the High Energy Physics community. These distributions are based on global analyses of deep inelastic scattering data and the regularly updated fits incorporate the latest data as well as new theoretical developments in the heavy quark sector.
Nigel Glover, Adrian Signer, James Stirling
It is a property of QCD that quarks and gluons can never be directly observed and only colourless hadrons can be detected in the laboratory. When a high energy quark or gluon is produced in an experiment, it is observed as a spray or `jet' of colourless hadrons (such as the pion or the proton). We expect that QCD should explain the `jet' data since these short distance processes are governed by perturbative QCD.
Chris Maxwell, Adrian Signer, James Stirling
Alan Martin, James Stirling
There is a long history of determining the partonic (quark and gluon) structure of the proton from deep inelastic and related hard scattering data. If Bjorken x is sufficiently large the parton distributions satisfy DGLAP evolution equations which resum the (alphaslogQ2)n contributions. However experiments at the electron-proton collider, HERA, at DESY have opened up the small x domain x<~10-3. In this region it is necessary to sum up the (alphaslog(1/x))n contributions. At leading order this is accomplished by the BFKL equation. Durham researchers have made many contributions to the understanding of the underlying dynamics of the processes at low values of the Bjorken x, from before the advent of HERA to the present day.
Valery Khoze, James Stirling
The distribution of soft hadrons or jets accompanying energetic final-state particles in hard scattering processes is governed by the underlying colour dynamics at short distances. The soft hadrons paint the colour portrait of the parton hard scattering, and can therefore act as a `partonometer'. Since signal and background processes at hadron colliders can have very different colour structures (compare for example the s-channel colour singlet process q qbar -> Z' -> q qbar with the colour octet process q qbar -> g* -> q qbar), the distribution of hadronic flows in the events can provide a powerful additional diagnostic tool for identifying new physics processes.
Diffraction may be regarded as the elastic scattering of particles (or constituents), which has its origin in the optical theorem. For example, a diffractive process can be p p -> X Y, where X and Y are either the elastically scattered proton, or one of its low-mass excitations, or, at higher-mass, a group of hadrons coming from the dissociating proton.
The classical method of detection is to tag the diffracting system. However in high energy collider experiments, the elastically scattered protons or N*'s disappear into the beam pipe and require special detectors far down, and adjacent, to the pipe. Alternatively the rapidity gap method is often used, which occurs because the scattered p or N* or X travels roughly in the same direction as the original hadron, leaving a large gap between the rapidity of this `particle' and the other emitted particles. For soft interactions the processes are characterised by (Regge) Pomeron exchange, whereas hard diffractive interactions are mediated by hard or QCD `Pomeron' exchange, which at lowest order is represented by two-gluon exchange.