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Theory & Phenomenology

20 May 2024

Theory & Phenomenology

Standard Model Physics

Parton distribution functions

Parton distribution functions (PDFs) describe the energy distribution of partons inside nucleons and therefore the flux of colliding partons in experiments such as the Large Hadron Collider (LHC). They are therefore a fundamental ingredient in essentially all forms of theoretical predictions for the LHC and the interpretation of its data. While this work is concentrated on LHC phenomenology it has important applications in other areas of HEP e.g. in studies of ultra-high energy neutrinos, like those being carried out by other UCL experimental researchers.

Parton density functions showing the probability to find a given quark or gluon species inside the proton, at a momentum transfer scale Q, as a function of the fraction x of the proton energy carried by the parton.

The UCL group is largely responsible for one of the main PDF sets (MMHT, previously MSTW) used as standard by experimental groups and theoretical particle physicists. The 2008 update of the MSTW PDFs is the most highly cited paper in theoretical particle physics since 2009, and the more recent update MMHT2014 is now being used for the same purposes. As well as producing the PDFs the UCL group is involved in theoretical developments in QCD calculations, in particular involving heavy flavour, resummations and central exclusive production.

Monte Carlo simulations for colliders

This kind of theoretical modelling plays a role in everything from the initial design of the experiment and the detector calibration, to, most intensively, the understanding of the data and its interpretation. In particular, Monte Carlo (MC) simulation programs are intended to realise a very wide range of our current theoretical knowledge as life-like event generators, as precisely as possible, in order to be as sensitive as possible to deviations from it in the data. Such deviations may then be analysed as `new physics' where, again, MC event generators have a central role to play in extracting information about whatever that may be.

UCL is heavily involved in the development of Monte Carlo simulations of LHC collisions and has contributed to two of the main programs in use there (Herwig++ and The Powheg-Box). As well as working in the construction and maintenance of simulations for our experimental colleagues, UCL has also contributed to theoretical developments in this field, taking these generators to the next levels of accuracy.

Monte Carlo simulation of Higgs boson production and decay to leptons (isolated yellow and orange tracks) in association with multiple hard QCD jets (red).

Most recently the UCL group, together with friends in Milan and Oxford University, has developed a state-of-the-art precision simulation of Higgs boson production, including exact next-to-next-leading-order QCD corrections.