Theoretical Particle Physics Rolling Grant

In order to reach the highest possible energies, the current particle colliders, the DESY electron-positron collider at Hamburg (which finished running in 2007, but from which data are continuing to appear) and the Tevatron proton-antiproton collider near Chicago, use protons as at least one of their colliding particles. Protons are particles which interact via the strong force, and are composite particles because the strong force binds the fundamental constituents, partons - which may be quarks or gluons. The collider due to turn on at CERN, the Large Hadron Collider (LHC), will be a proton-proton collider. The LHC will achieve the highest energies at a particle collider by a factor of more than 7, and will enable us to see if the missing particle within the Standard Model of particle physics, the Higgs boson, exists - as well as to detect the first signs of physics beyond the Standard Model, for example Supersymmetry, where each Standard Model particle has a supersymmetric partner. At low energies we think of the partons as being bound within the proton, but at very high energies the strong coupling becomes weaker, and the interactions of colliding protons can be thought of as interactions between the partons in each proton, which may be calculated as an expansion in the strong coupling constant. Hence, in order to understand the results of any hadron collider experiments one must first understand how the proton is made up out of its partonic constituents. To a certain degree this can be calculated, but the strong coupling makes some expansions badly defined, and some of the information must be determined by comparison with experimental data. Therefore, one must perform enough independent experiments, and use the theoretical calculations within the theory of the strong force (Quantum ChromoDynamics - QCD) as accurately as possible, to determine the composition of the proton in terms of the gluons and the six flavours of quark (up, down, strange, charm, bottom and top). This requires the use of data from a variety of experiments, and it must be checked that all pieces of data are consistent with the partons and the QCD theory, hence testing QCD to great accuracy, and measuring the strong coupling. Once a consistent set of parton distributions is determined, these partons may be used to predict any other process using the protons. This can be the production of beyond the Standard Model particles, or for Standard Model processes, where the latter often mask the former. The project proposed is to improve the determination of parton distributions and their consequences for collider physics. This will be achieved by incorporating new theoretical calculations, e.g. higher order in the coupling or more precise inclusion of heavy particle corrections, and also by the inclusion of data on new processes, both from the existing colliders and from the forthcoming LHC. These new theoretical developments and the new influx of data will mean we require a lot of effort to obtain the best partons. However, this is essential if the data are to be interpreted properly, and hence, if we are to increase our understanding of the Standard Model and also to search for the physics beyond it. It is very likely that any signal for new physics at the LHC will initially be ambiguous, since it could be due to an uncertainty in our understanding of the Standard Model, in particular strong interaction physics and parton distributions, and this has previously occurred at other hadron colliders. The group at UCL has the precise expertise to disentangle these possibilities due to both the experience in analysing numerous different types of data sets in comparison to predictions, and in developing improvements to the theoretical framework. In the case that a new signal is observed, the group will aim to help interpret precisely what it signifies, where again the ability to separate it out from the background will be vital.