Particle Physics Phenomenology

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.

Both areas of Research at UCL have potential impact which will be summarised in turn. Let us begin with the Standard Model research. The extraction of the MSTW parton distribution functions, which quantitatively describe the manner in which the proton is made up of its constituent quarks and gluons, is led by Thorne. This has a variety of different areas of impact. Since it is one of the major pieces of information which goes into obtaining both the most likely values and the uncertainty on the rates of production of particles, both known and hypothesised, at particle colliders it has an impact on decisions on the energy at which these colliders should be run, when breaks should be taken, and indeed, whether colliders should be continued or switched off. As an example, the MSTW parton distribution functions were the single set which were used over the past two years to determine the limits on Higgs boson masses at the Tevatron collider at Fermilab, and to deduce what improvement in limits, or significance on signal could be found in the future. Similar reasoning is true for the reaction rates for ultra-high-energy neutrinos or other cosmic rays, so will impact on the considerations in planning the large scale detectors for these. Hence, the work at UCL will impact upon the nature of decision making in large-scale science projects, in particular the LHC in the next few years, in both the UK and worldwide.
Page 4 of 10 Date printed: 02/02/2011 15:11:01
ST/J000515/1 Date saved: 02/02/2011 10:18:35
Additionally, the manner in which the proton (and neutron) are made up from quarks and gluons, and hence the details of the strong interaction force are also one of the fundamental questions of particle physics. As such the results of the MSTW analyses are likely to make their way into textbooks and the popular science literature as the best illustration of this particular, but rather fundamental aspect of physics. Indeed, Thorne is due to write the article describing this physics for the peer-reviewed version of Wikipedia, known as Scholarpedia, so the default online source for the account of this subject will be extremely directly influenced by the work. The BSM physics research led by Deppisch also has significant potential impact. While neutrinos physics aspects have proven to be a major source of new discoveries, the public is not sufficiently aware of their importance. As neutrinos encompass a wide range of aspects such as particle physics, astrophysics and cosmology, they have the potential to play a key role in the public appreciation of physics research. There is the intention to undertake several measures to publicly communicate the research results in particular, and promote neutrino physics in general. This will include: (i) creation of a public internet blog, which will regularly inform on relevant physics developments; (ii) participation in university events such as open days, along with the creation of information materials; (iii) work with schools, offering class materials and presentations; (iv) collaboration on public outreach with H. Paes (U. of Dortmund), who will publish a popular science book on neutrino physics next year, which, in part, is based on past research with Deppisch. In addition, UCL has a wide range of training courses on public communication, and the intention is to use these to the fullest to enhance skills in public outreach. The planned development of a general software package for particle physics analysis by Deppisch is largely motivated by the desire to systematically assess the sensitivity and discriminative power of existing and planned experimental efforts with respect to fundamental physics searches. Such an assessment is a crucial step of the scientific decision making and steering process. As such it is expected to have an important impact in helping to influence recommendations for future particle physics efforts. This is especially topical given the current economic climate and funding situation, and would help to optimally distribute resources.