Particles, Fields and Extended Objects

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The STFC research programme of the Theoretical High Energy Physics Group atCambridge University is focused on the fundamental problems of colliderphenomenology, quantum field theory, string theory and gravity, and analysing aclass of strongly interacting particles called hadrons.In this research, weshall perform calculations to understand the fundamentals underlyingreality and our understanding of the universe and matter within it. Much of this effort supports particle physics experiments at CERN andelsewhere, as well as astrophysical and cosmological observations of theuniverse.Technical, difficult and detailed calculations deep in the quantum theory arerequired in order to interpret some of the experimental data and to learneverything we can from them. The structure of the proton (the particlescollided at the Large Hadron Collider) will be understood better in order to getrobust and reliable predictions on the collisions. We are analysingand interpreting Large Hadron Collider data from CERN to do various things: looking for signs of new particles or forces, developingsearch and measurement strategies for them, or making high precisionpredictions of various theories. The Standard Model is the current model ofparticle physics that is well accepted, verified and measured. Most of itspredictions agree well with collider data. However, it leaves many questionsunanswered: why do the fundamental particles have the particular pattern theydo in their masses? We shall be developing mathematical models, based oncurrent data, to try to explain some such features, and provide experimentaltests at the same time. We are also busy supporting the science case for futurecolliders, investigating which questions they could answer well.How gravity behaves at small distance scales is badly understoodtheoretically, although string theory may be an interesting framework forunderstanding it.We will be developing and investigating theories of quantumgravity mathematically in order to push the understanding forward.Blackholes provide a particular focus for the calculations: these are objectsaround which gravity is very strong, and we will learn much from theirtheoretical study. Variouscalculations in new developments of string theory are important for this, andfor the development of how to calculate particle scattering in general. String theories will be constructed to see how close they come to the universewe see. Also, models of inflation (a time in the early universe when theuniverse underwent extremely rapid expansion) will be investigated, developedand compared with observations.Some particles, such as hadrons, are strongly bound states of smaller ones. For these, sophisticated computer programs are built which break space and time up into a grid of points, and the quantumfluctuations of the sub-nuclear interactions are simulated using randomnumbers on this lattice. Analytic calculations must be done to match thenumbers obtained on the computer to experimental data. We shall develop these calculations, and perform new ones so that data can be used to extract the level to which various quarks (for example, the up quark and the b-quark) mix. This helps provide an accurate description of an unexplained phenomenon: how the funny pattern of quark mixing comes about. These calculations also help the extraction of the difference between matter and anti-matter from experimental data. We can predict much about which strongly bound states may exist and their properties, and studies of the more exotic and puzzling varieties seen in experiment will be an important avenue of work.