Theory Consolidated Grant. - Standard Model Phenomenology and Beyond the Standard Model Phenomenology.

The main focus of Hamilton's research is to further develop precise computer simulations based on QCD calculations (so-called Monte Carlo techniques) for the large number of final state particles in collider physics processes. This will facilitate discovery and interpretation of new physics at the Large Hadron Collider (LHC). While precision simulations may not always be needed to claim a discovery, e.g. if new physics reveals itself as a heavy, easily resolved new particle, in many scenarios, such as supersymmetry, signals are expected to manifest as subtle distortions in the shapes of distributions. Hence, an accurate understanding of the Standard Model background, subject to all experimental cuts, is fundamental to claiming a discovery, or setting exclusion limits. Moreover, precise simulations are essential in attempting to determine what it is that has been found. The newly discovered 125 GeV mass Higgs-like boson is a prime example of this, and one for which simulations that Hamilton contributes to are being used to this end by the LHC Higgs working group. Research in the near future will focus on improved precision and theoretical rigour for the Monte Carlo generators used in simulations.

Thorne's work is complementary and in many senses similar. It involves the details of the initial state in particle collisions. At colliders which use hadrons, e.g. the LHC which uses protons, the beam is effectively made up of the hadronic constituents, quarks and gluons, generically known as partons. Hence, in order to make predictions for any reaction, both for Standard Model and new physics, one needs to know the precise partonic composition of the hadrons in terms of both the energy fraction of the hadron and the energy scale of the scattering process (e.g., the mass of a particle produced). Thorne is the lead member of the MSTW group that provides one of the standard sets of parton distribution functions (PDFs) used in both the experimental and theoretical analyses at the LHC and previous colliders. The technique is constantly being improved in terms of the theoretical basis, and more data are appearing which help constrain the PDFs further. Hence, Thorne's work will be based on improving PDF determination, providing updated PDFs for use at colliders and helping to determine the consequences of any changes in both central values and uncertainties. Both Hamilton and Thorne are involved in precision calculations at the LHC and will provide expertise in interpreting any deviations which could be the first sign of Beyond the Standard Model (BSM) Physics.

BSM phenomenology aims to uncover deeper laws and structures in nature than currently known. The work of Deppisch achieves this by taking experimental results (e.g. from the LHC) and comparing them with predictions derived from new theoretical ideas (e.g. the notion of more than three space dimensions). Neutrinos play a very important role as they are the least understood of all known matter particles. Their most mysterious property is their lightness; the exact value of the mass is unknown but it is at least a million times smaller than the next lightest particle, the electron. There is no agreed explanation for this huge discrepancy which goes at the heart of the fundamental question: What is mass? Even if the Higgs boson is confirmed by the LHC as the source of the mass of particles such as quarks, the lightness of neutrinos still remains a mystery, but we also expect that once we solve this, a large piece of the puzzle of nature will fall in our lap. The proposed research aims to determine the absolute neutrino mass as precisely and robustly as possible from a range of experimental results which are expected over the next years. In addition, the work will correlate physics phenomena at different experiments within a theoretical framework. This is necessary as no single experiment can probe all aspects of nature.

We begin with the impact of the Standard Model research. The extraction of the MSTW parton distribution functions (PDFs) impacts in a variety of areas. PDFs are a major piece of information required for obtaining both the central values and uncertainties on the rates of particle production at colliders. Naturally, therefore, they impact on decisions concerning the energy at which colliders should be run, when breaks should be taken, and indeed, whether they should be continued or switched off. The MSTW PDFs were the single set used to determine the limits on Higgs boson masses and significance of excesses at the Tevatron collider at Fermilab, and to deduce what improvement in limits/signal significance could be reached with future running. Parton luminosity plots provided by the MSTW collaboration are one of the standard pieces of summary information used in meetings for future planning. Similar reasoning is true for the reaction rates for ultra-high-energy neutrinos or other cosmic rays, so PDFs will impact on the planning of large scale detectors for these. Additionally, the manner in which the proton is comprised of quarks and gluons is one of the fundamental results of particle physics. As such, the results of the MSTW analyses are likely to make their way into textbooks and popular science literature as a standard illustration of this particular aspect of physics. Indeed, Thorne is due (after some delays) to write an 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, and will be periodically updated as important new results are found. Naturally, as with the PDFs, besides theoretical and phenomenological studies, Monte Carlo event generators have an important role to play in the planning and design of future experiments, and in determining the way in which existing experiments are commissioned and run. Indeed, prominent documents on these subjects, such as the ATLAS technical design report, are heavily comprised of simulation-based studies. The design and physics programs of current and future experiments, most notably the LHC, will therefore be directly influenced by the tools developed by Hamilton and collaborators, e.g. the Herwig++ and Powheg-Box packages. Hence, Standard Model work at UCL will impact upon decision making in large-scale science projects in both the UK and worldwide.

Being a part of fundamental research, the Standard Model and particularly the Beyond-the-Standard-Model (BSM) phenomenology research serves the purpose of deepening human knowledge of the fundamental properties of nature beyond the current understanding, and is therefore beneficial to the general public. Such fundamental research is especially attractive to young people, and can serve as a gateway to enter a career in science, technology, engineering or mathematics, areas that are vital to the economy. This effect will occur both indirectly by disseminating the research results of the project to the wider public, as well as directly by training students who may choose careers outside fundamental research. The specific BSM research proposed here is performed in collaboration with experimental colleagues of the neutrinoless double beta decay experiments SuperNEMO and SNO+ supported in the UK, and its aim is to clearly understand the impact of ongoing and future experiments on fundamental theories. It therefore helps in evaluating the benefit of experiments with regard to their costs, which is necessary for science policy-making agencies both at national and international level. Future experimental efforts are directly assessed by similarly determining the impact of planned or hypothetical experiments on the physics results that can be extracted. This will prove helpful as well for science policy-making agencies when deciding on the composition of future experimental efforts.