Tuning of Monte Carlo Event Generators

Physics at the Large Hadron Collider (LHC) is likely to involve a bewildering array of new phenomena, due to new heavy particles which have never been produced before with a man-made particle collider. At present, we have no idea what the characteristics of these particles will be, but there are many theoretical models which predict particular arrangements. These include such exciting ideas as 'supersymmetry', which at least requires a doubling of the number of known fundamental particles; the existence of extra dimensions, which would be a deeply profound discovery; or a menagerie of other variations. Any of these, if discovered, would be a giant leap forward in our understanding of the universe. Much of LHC physics will be geared towards working out which, if any, of these models is realised in nature. Part of the reason that LHC physics will be difficult is that the phenomena that particle physics has spent the past 50 years investigating to such high accuracy - the extraordinarily successful 'Standard Model' (SM) - will be a huge source of 'background noise' in measurements of new physics. To study the new, heavy particles, we must first have excellent control of the SM physics which threatens to hide them. If our models of the SM are not accurate, it may be impossible to extract clean signals of the new particles. Or, perhaps worse, we could accidentally claim the discovery of something which is not actually there! Understanding how the SM manifests itself in collider experiments is not easy: in particular, the strong nuclear force is problematic when it comes to making calculations. This force 'glues together' fundamental 'quark' particles, which are never seen in isolation, to make the observed 'hadron' particles like protons and neutrons. Calculations made using QED, the SM theory of electromagnetism, rapidly converge to accurate predictions, but the corresponding calculations in QCD, the strong force theory, are extremely complex and the convergence rate is much slower. There are also aspects of QCD which cannot be handled by normal methods: for example, the process by which quarks bind strongly into hadrons is not well understood. To get around these calculational problems, particle physicists use computer simulations which, rather than calculating everything from the most fundamental QCD ideas upwards, take a more approximate path. These simulations, called 'event generators', simulate all aspects of particle collisions, from the most energetic interactions, to the spray of particles that come from a high energy quark, and to the eventual low-energy conversions of quarks into hadrons. The price that one must pay for such a calculational shortcut is predictivity: where a genuine QCD calculation, albeit one massively beyond any known methods, would only require a few numbers to be specified, event generators have a lot of input parameters. The only real test of whether or not the values given are 'good' is how well the event generator predictions compare to real measurements. This is of enormous importance to LHC physics: unless the QCD backgrounds to new physics are understood, in the form of reliable event generator tunings, all the new and exciting physics may remain hidden. With so many parameters, however, working out which combinations are good is a substantial task in itself. The aim of this project is to systematically tune all the major event generators to be used at the LHC against all the available data, so that QCD backgrounds to new physics are well-modelled. It is important that the data against which the generators are tuned includes that from the LHC itself, as areas of QCD which have never been probed before will be encountered there. With a good control of QCD models, the new physics which the LHC was built to find can be reliably identified and studied, perhaps changing our view of the universe forever.