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LHCTHEORY Report Summary

Project ID: 291377
Funded under: FP7-IDEAS-ERC
Country: Switzerland

Final Report Summary - LHCTHEORY (Theoretical predictions and analyses of LHC physics: advancing the precision frontier)

The primary goal of the LHCtheory project was the realization of a complete automatic framework for the simulation of particle collisions at high energy, including the most precise available calculations for the description of any arbitrary parton-level process and of its full evolution into physical observables. This task includes several independent elements, which formed the various work packages: the calculation of partonic cross sections for Standard Model (SM) processes, including next-to-leading order (NLO) QCD corrections; the calculation of cross sections for processes involving interactions of theories beyond the SM (BSM), and the inclusion of NLO corrections for these processes; the calculation of NLO electroweak corrections to the various processes; the calculation of NLO corrections due to the exchange of new particles appearing in BSM theories; the calculation of corrections to SM processes induced by the presence of higher-dimension operators, parameterizing in an effective field theory framework the existence of new phenomena at large energy scales. All of these tasks have been achieved in the course of the project. With these elements in place, additional technical steps, essential to complete the overall framework and make it suitable for realistic simulations, usable by the experiments to interpret their data, became possible in a fully automatic manner: the matching of the NLO parton-level calculations to a full-fledged evolution of the partonic shower and its eventual hadronization; the consistent merging, at full NLO, of monte carlo data samples describing events of increasing jet multiplicity. Once again, these results were achieved in the context of a framework that allows, by simply defining the desired final state, to dispatch to a numerical code the task of organizing and preparing the various elements of the required calculation, and carry out the relative numerical analysis.
The net outcome of this progress is the code MG5_aMC@NLO. This has become a leading tool used by the LHC experiments for the description of a vast array of phenomena, from SM processes, to the production of BSM particles, to the study of reactions involving the Higgs boson. The thousands of citations collected in the past few years by MG5_aMC@NLO and its ancillary products testify the immense success of the project. In addition to the use by the experiments, a large number of phenomenological studies have been completed in the context of the LHCtheory project, and by the community of theoretical physicists at large. The phenomenological exploitation of MG5_aMC@NLO was one of the key deliverables of the LHCtheory project. Studies have been performed of a large variety of processes, exploring the implications for LHC physics, identifying useful observables for precision measurements, studying the backgrounds for the searches of new physics and for the determination of Higgs properties. Tools have been developed to streamline the use of the MG5_aMC@NLO results in the fitting of the parton density functions (PDF) of the proton, comparing the calculations to the LHC data for observables sensitive to the full structure of the final state, such as jet spectra.
A further, very ambitious, research item announced in the project proposal was the extension of NLO calculations to the following perturbative order, namely to next-to-next-to leading order (NNLO). A whole new technique, based on the regularization in four dimensions of the ultraviolet divergencies affecting loop calculations (FDR), has been developed within the project by the University of Granada team of the collaboration. FDR has been successfully extended to NNLO, and work is ongoing to automate its use. The development of general automatic techniques to address the full complexity of NNLO processes in QCD will still require a lot of work. Students grown under the LHCtheory project, continue to tackle the challenge after having graduated and undertaken an independent research career.

The results obtained during the project have furthermore opened new avenues of research and created new prospects for progress. To name a few, the development and automation of an NNLO subtraction scheme, extensions of the OPP method to 2 loops, and a full-fledged setup for the analysis of DM signals in direct and indirect detection and in collider production. Other important areas are in progress, like the automatic calculation of loop-induced processes at NLO, and the matching of NLO EW corrections with the parton shower. On the technical level, R&D has started on new techniques for large-scale event generation via the Grid, and for numerically efficient event reweighing, to facilitate the production of large monte carlo datasets necessary for future runs of the LHC.

As of May 15 2017, the Inspire HEP data repository lists 97 papers published in referee journals acknowledging the support of the LHCtheory grant. These have collected a total of 6876 citations, for an average of over 70citations/paper, and an h-factor=30. Including papers appeared as preprint or as reports of working groups, these figures grow to 127 papers, 8707 citations and h=37.

In summary, the project has delivered all of its promised results, and has paved the way to further, not anticipated, developments, which are currently being explored. The ERC grant has contributed to the realization of a research tool that has become an indispensable element for the full exploitation of the rich data collected by the LHC experiments.

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