Particles, Fields and Extended Objects

The STFC research programme of the Theoretical High Energy Physics Group at Cambridge University is focused on the fundamental problems of collider phenomenology, quantum field theory, string theory and gravity, and analysing a class of strongly interacting particles called hadrons. In this research, we shall perform calculations to understand the fundamentals underlying reality and our understanding of the universe and matter within it. Much of this effort supports particle physics experiments at CERN and elsewhere, as well as astrophysical and cosmological observations of the universe. Technical, difficult and detailed calculations deep in the quantum theory are required in order to interpret some of the experimental data and to learn everything we can from them. The structure of the proton (the particles collided at the Large Hadron Collider) will be understood better in order to get robust and reliable predictions on the collisions. We are analysing and interpreting Large Hadron Collider data from CERN to do various things: looking for signs of new particles or forces, developing search and measurement strategies for them, or making high precision predictions of various theories. The Standard Model is the current model of particle physics that is well accepted, verified and measured. Most of its predictions agree well with collider data. However, it leaves many questions unanswered: why do the fundamental particles have the particular pattern they do in their masses? We shall be developing mathematical models, based on current data, to try to explain some such features, and provide experimental tests at the same time. We are also busy supporting the science case for future colliders, investigating which questions they could answer well. How gravity behaves at small distance scales is badly understood theoretically, although string theory may be an interesting framework for understanding it. We will be developing and investigating theories of quantum gravity mathematically in order to push the understanding forward. Black holes provide a particular focus for the calculations: these are objects around which gravity is very strong, and we will learn much from their theoretical study. Various calculations in new developments of string theory are important for this, and for the development of how to calculate particle scattering in general. String theories will be constructed to see how close they come to the universe we see. Also, models of inflation (a time in the early universe when the universe underwent extremely rapid expansion) will be investigated, developed and 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 quantum fluctuations of the sub-nuclear interactions are simulated using random numbers on this lattice. Analytic calculations must be done to match the numbers 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.