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 class of strongly interacting particles called mesons. 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 is the Higgs boson so light (the theory predicts it should be 10^15 times heavier)? what is dark matter? how come the universe is made of matter and not anti-matter? Models of new physics explain some or all of these, and typically predict new particles. Finding these (or ruling them out) is a priority in order to test such theories. The Large Hadron Collider has just upgraded to its highest energy, 13 TeV, which means that heavier particles may be found that haven't been seen before. There have been exciting unexpected "bumps" in data recently: for example too many pairs of particles of light (photons) seem to be coming out of the proton proton collisions with an energy equivalent to 750 proton masses. If it is verified, this would be a signal of a new particle which decays to two photons, and the question is: where does this fit and what does it mean? We are actively working on such questions. Quantum field theory provides a very successful description of known particle interactions. However, special techniques are required to get predictions when the interactions are strong. We shall be developing various techniques to improve these, and to provide understanding of the underlying dynamics. String theory is an extraordinarily mathematically rich structure, that purports to describe gravity. One variant of it may also even underlie all of the interactions between particles that are observed in nature. The tiny loops behave like particles unless one probes them at energies that are far too high for us to reach in current experiments. Some of our research examines the rich structure behind the mathematics of these theories: it turns out that scattering two particles and scattering three particles have strict relations between the interaction probabilities. Sometimes, truths such as these are easier understood by mapping one string theory to another one, which has a different coupling strength and a different number of space-time dimensions. These "dualities" help us winkle out truths and deep connections in string theory. We shall be investigating their role in the relations between interaction probabilities. We are analysing instabilities in theories of black holes, depending on the number of dimensions and how bent the underlying space-time is. Some particles are strongly bound states of smaller ones, such as B-mesons. 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.