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 analysingclass of strongly interacting particles called mesons.We are analysing and interpreting Large Hadron Collider data from CERN to dovarious 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 is the Higgs boson so light (the theory predicts it should be10^15 times heavier)? what is dark matter? how come the universe is made ofmatter and not anti-matter? Models of new physics explain some or all ofthese, and typically predict new particles. Finding these (or ruling them out)is a priority in order to test such theories. The Large Hadron Collider hasjust upgraded to its highest energy, 13 TeV, which means that heavierparticles may be found that haven't been seen before. There have been excitingunexpected "bumps" in data recently: for example too many pairs of particlesof light (photons) seem to be coming out of the proton proton collisions withan energy equivalent to 750 proton masses. If it is verified, this would be asignal 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 suchquestions. Quantum field theory provides a very successful description of known particleinteractions. However, special techniques are required to get predictions whenthe interactions are strong. We shall be developing various techniques toimprove these, and to provide understanding of the underlying dynamics.String theory is an extraordinarily mathematically rich structure, thatpurports to describe gravity. One variant of it may also even underlie all ofthe interactions between particles that are observed in nature. The tiny loopsbehave like particles unless one probes them at energies that are far too highfor us to reach in current experiments. Some of our research examines the richstructure behind the mathematics of these theories: it turns out thatscattering two particles and scattering three particles have strict relationsbetween the interaction probabilities. Sometimes, truths such as these areeasier understood by mapping one string theory to another one, which has adifferent coupling strength and a different number of space-timedimensions. These "dualities" help us winkle out truths anddeep connections in string theory. We shall be investigating their role in therelations between interaction probabilities. We are analysing instabilities in theories of black holes, depending on thenumber of dimensions and how bent the underlying space-time is. Some particlesare strongly bound states of smaller ones, such as B-mesons. For these, sophisticated computer programs arebuilt 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 thesecalculations, and perform new ones so that data can be used to extract thelevel 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 alsohelp the extraction of the difference between matter and anti-matter fromexperimental data.