UNIVERSITY of GLASGOW

Particle Physics Theory
Schematic view of top quark production at the Tevatron

Perturbative QCD

Since the current generation of high-energy experiments use colliding beams of protons (or anti-protons) it is especially important to understand the sector of the SM that accounts for the strong interaction, the theory of Quantum Chromodynamics (QCD). This theory describes strongly-interacting particles such as protons in terms of the elementary quarks and gluons of which they are composed. Although this theory cannot be solved exactly, it is possible to develop approximate predictions in the form of perturbative expansions.

The expansion is in terms of the strong coupling (&alphas) which is approximately 0.12 at energies in which we are interested. Each successive term in the expansion is suppressed by one power of &alphas. Many calculations have been performed using the simplest predictions from the first, or lowest order, term in this series. Since the strong coupling is quite large (much larger than the corresponding electromagnetic coupling, which is about 1/137), such calculations have only limited accuracy.



Summary of measurements of the strong coupling, alpha-s



In order to produce accurate theoretical predictions to compare with experimental results, it is preferable to calculate higher order terms in the perturbative series. These calculations do not just provide more accurate predictions of cross sections and event rates. They also begin to model more accurately the sub-structure of the colliding hadrons and the production of jets of particles observed in the detectors. Unfortunately, performing such calculations is a difficult task and these predictions only exist in a limited number of cases.






Summary of measurements of the strong coupling, alpha-s

Research at Glasgow

In collaboration with other theorists around the world, researchers at Glasgow are developing these more sophisticated predictions from perturbative QCD. With them, we can investigate the phenomenology of particle collisions in more detail and help to understand the many types of events that we will observe at the LHC in 2008.

An example of this work is the Monte Carlo program MCFM, which implements many next-to-leading order (NLO) predictions for hadron-hadron colliders such as the Tevatron and the LHC. It has already been used to successfully describe data in many analyses by the CDF and D0 collaborations at the Tevatron.

The emphasis of recent work has been on the calculation of SM backgrounds for processes which are of particular importance. Examples include the recent consideration of final states which contain a W or Z boson together with one or more bottom quark jets. Such events provide a very large background to single top quark production (yet to be definitively observed at the Tevatron) and to Higgs production in association with a W or Z boson. Research is ongoing into further calculations that will be necessary in order to better interpret experimental colleagues' analyses at the LHC.

References