Particle Phenomenology, QCD and the Standard Model.

Keith Hamilton's research concerns the development of high precision Monte Carlo simulations of collider Physics processes. These facilitate discovery and interpretation of new physics at the Large Hadron Collider (LHC). While precision simulations may not be needed to claim a discovery, e.g. if new physics appears as a clear `bump' in a smooth distribution, (as did the Higgs boson), in many scenarios, such as supersymmetry, signals are expected to manifest as subtle distortions in the shapes of distributions. An accurate understanding of the Standard Model background, subject to all experimental cuts, is then unavoidable for claiming a discovery, or setting exclusion limits. Precision simulations are also essential to determine what it is that has been found. The recently discovered 125 GeV mass Higgs boson is the prime example of this. In 2012-2013 Hamilton and collaborators in Milan and Oxford developed the world's most accurate simulation of Higgs boson production. This simulation is used by the ATLAS collaboration, appearing in their publications confronted with measurements which test various Higgs boson properties. Hamilton plans to fully exploit the innovations which led to the latter simulation, applying them to produce
precision simulations of other important LHC reactions, in particular the W-pair and diphoton production processes, where anomalies have been reported in recent experimental measurements. In tandem with application of these new techniques he has helped develop, Hamilton also plans to continue to press on with further innovations in the theoretical frameworks they are based on, towards greater accuracy.

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 MMHT (previously 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, and in high energy neutrino and cosmic ray physics. 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 elsewhere) and helping to investigate the consequences of any changes in both central values and uncertainties. Thorne is also a core member of the PDF4LHC working group (being a member of the steering committee since the inception, and hosting the website) which provides recommendations for use of PDF sets at the LHC.

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.

The extraction of the MMHT parton distribution functions (PDFs) impacts in a very 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 any colliders with protons (or antiprotons) in the intial state. 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, and now MMHT collaboration are one of the standard pieces of summary information used in meetings for future planning. Most recently planning is often based on the PDF4LHC prescriptions for use of PDFs based on a combination of a small number of different sets, including MMHT, and where Thorne is a core member of the PDF4LHC committee. In fact the PDF4LHC website, the home of the recommendation, is based at UCL and maintained by Thorne. Similar reasoning to that for particle colliders is also 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 MMHT 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 remains scheduled (following delays due both to lack of time and the rapidly changing situations with PDF over the past couple of years) 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.

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 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. Some of our recent students and RAs have decided to leave the academic world and have entered into both financial and industrial areas of work, using the very sophisticated mathematical and computational techniques they have learned and developed from their research work, and there is a high likelihood this will continue to some extent in the future.