Particle Physics Phenomenology

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The particles called hadrons - e.g. the proton and neutron, of which atomic nuclei are composed - are actually made up of strongly interacting quarks and gluons (partons). A scattering process for a hadron is characterised in terms of two variables, x, the fraction of the momentum of the hadron carried by the parton and the energy scale of the process. Hadron interactions are described in terms of individual parton interactions and summed to give the total interaction rate. Parton interactions are calculable as an expansion in the QCD coupling constant, which becomes weaker at high energies. The make-up of the hadron in terms of partons cannot be directly calculated, but the variation with the energy scale can be if the scale is high enough for the coupling to be small. Hence one requires input parton distributions at low scales, which have to be extracted from experiment, then results of subsequent experiments with initial state hadrons can be predicted. A study of partons increases our knowledge of the proton and is an excellent test of QCD, and is also essential in searches for new particles. Much of current experimental particle physics involves hadron colliders, in particular the LHC at CERN which will hopefully for discover the final particle of the Standard Model, the Higgs boson, and also new physics. Understanding the LHC results to high accuracy and being able to disentangle the signs of new physics necessarily requires detailed knowledge of parton distributions and QCD interactions. One of the main parts of this project is obtaining partons from existing data and using theory, and developing the required theory. As well as finding the best parton distributions it is important to understand the uncertainties, both experimental and theoretical, on the processes calculated using partons at colliders, and this will be done at UCL. The theoretical uncertainty is the more difficult task, requiring a good understanding of the wide range of corrections that may be applicable to standard perturbative QCD. As the LHC obtains more data it will be compared to the theory, simultaneously improving our understanding of QCD and leading to improved predictions. At some point discrepancies between predictions and measurements will require a very detailed investigation of the significance of the discrepancy and the likelihood of it being a signal of new physics. One of the main candidates is the Higgs boson, and understanding partons will be instrumental in deciding if the Higgs boson has been discovered, and if so, determining its properties. Neutrinos are an all present yet elusive part of the Universe: More than fifty trillion neutrinos produced in the Sun pass through one's body per second without leaving a trace. They play an important role in most of the phenomena in the Universe, from the energy production in stars and supernova explosions to the general structure of galaxies. As such they currently provide the only solid departure from what is considered the Standard Model of particle physics and an explanation of their properties forms a crucial part of new theories that aim to understand the fundamental structure of nature. Because of their tiny masses, neutrino properties will be hard to detect at the LHC, but in new physics models there is often an interplay between predictions of neutrino properties and observations at the LHC. Given the role of neutrinos as a 'missing link' in our understanding and the potential of the LHC to provide completely new insights, this project aims to exploit the interplay between these two physics aspects to probe the fundamental laws of nature. The project therefore directly addresses the following question: What can we learn about fundamental models by combining LHC and neutrino data? Ultimately, it therefore aims to help in the understanding of the fundamental physics of nature.