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Establishing Precision and Controlling Uncertainties for Multijet Simulations at the LHC

Final Report Summary - PECULIJETS (Establishing Precision and Controlling Uncertainties for Multijet Simulations at the LHC)

The general aim of the project was to establish state-of-the-art next-to-leading order (NLO) QCD calculations as the default input to simulate hard scattering processes within the Herwig++ event generator, especially in the presence of additional jets in the final state.

The fellow has lead this effort within the Herwig++ collaboration, and has significantly contributed to all aspects of this general aim. Development and testing work towards the new member of the Herwig event generator family, Herwig 7, has been driven by the fellow and the Scientist in Charge, and made available both as the released package and as an accompanying publication in European Physical Journal C.

The package formed the basis for the project and is capable of simulating all standard model processes at NLO QCD, involving a technically limited, but in principle arbitrary number of jets. Processes involving photons can be simulated away from phase space regions sensitive to fragmentation contributions by using photon isolation criteria, which are explicitly available in the Herwig 7 release. Work towards the release also covered further development of and extensive testing of one of the shower modules, which where required to enable QED radiation in the angular-ordered shower and also a simplified implementation of NLO QCD matching. The fellow has developed and contributed to interfaces to external libraries, which could be used both at the matrix elements squared and amplitude level. Interfaces for the latter are supported through two distinct colour bases. The Herwig 7 release in all of the several aspects connecting to the project's objectives has been the main focus during the first year of the fellowship.

As envisaged by the proposal the fellow has been participating at the Les Houches workshop 2015, and has driven a number of studies and development related to the new Herwig release in this context, which has resulted in a total of four different contributions co-authored by the fellow to the Les Houches working group summary report. At the Les Houches workshop the fellow has also worked on an interface to the Recola package, which is specifically suited for obtaining the amplitudes relevant to QCD/QED mixed expansions as one of the steps towards including processes with photons, jets and QED corrections.

Within the first year of the project period, the fellow has started to address the relevant algorithms and code structures required for QED corrections, though one main focus has shifted to first exploit the structures from the new Herwig release as far as possible, and to ensure its timely application within the LHC experiments.

One of the opportunities taken up was the generation of loop-induced processes with the newly developed interfaces, which resulted in a phenomenological study co-authored by the fellow and published at the beginning of the second year in the Journal of High Energy Physics. In the first year, the fellow co-supervised a short-term student of MCnet ITN at Durham who's main project objective was to develop the steering and integration of Herwig within the LHC's experimental frameworks. The fellow also co-supervised a PhD student at Karlsruhe, to later join Durham as a postdoc, on work towards NLO multijet merging of multiple jets, a prerequisite to a later objective of the project.

This merging has been applied to the production of a Higgs boson and additional jets, and the fellow has also led work on NLO matched simulations for electroweak Higgs plus three jet production, and the production of a Higgs boson in association with top quark pairs all of which have been contributed to and published in the Yellow Report Number 4 of the LHC Higgs Cross Section working group.

In year two of the project the aspect of evaluating uncertainties of the simulation has become the main focus of the project. These have been addressed in great detail, and for both of the shower modules available in Herwig, as well as for processes including additional jets. The fellow has designed and led a comprehensive study, the first results of which have been published in a paper co-authored by the fellow to appear in European Physics Journal C.

The proposal's objective of combining jet production in the high energy limit together with a parton shower has been addressed from a new point of view; an algorithm has been identified which is based on the related development of re-weighting parton shower predictions on the fly. This algorithm, which has been developed by the fellow who also contributed and led its implementation in Herwig's shower modules, has been used for two applications: Reweighting for a fast evaluation of uncertainties, published in Physical Review D and co-authored by the fellow, as well as for generating so-called colour matrix element corrections set out earlier by the fellow, who supervised another MCnet ITN short term student working on this topic. Controlling this technology is also relevant to the project's goal of implementing and enhancing QED radiation in the parton shower algorithm, which has become available for the angular ordered shower module during the project's duration.

Uncertainties in matched calculations for the production of W boson pairs with two jets have been addressed by the fellow together with a collaborator at Hamburg who joined the Herwig collaboration as a specialist in VBF processes. The development of parton showers has become one of the fellows core objectives towards future development, and specifically the weighting algorithms, and the insight from colour matrix element corrections provide key ingredient to this future direction of research. Some aspects of this research have also triggered a new formulation of colour reconnection models, which will be included in a future Herwig release.

In all of these aspects the fellow has clearly benefited from the infrastructure and networks present at the host institution, and he has also kept up and further enhanced his collaborations with several other nodes of Herwig development and the community developing amplitude provider libraries. The fellow has also been trained on presenting the work, specifically the new Herwig release towards the end of the first project year, both at established workshops and mainly involving both theorists and experimenters, as well as at invited seminar talks. The fellow has explored new research directions and started to build up new collaborations geared towards a research programme which is building on his role of developing perturbative algorithms within the Herwig event generator, which has clearly been strengthened through the fellowship.

The fellow's career prospects have been enhanced, and several grant applications are currently under review both at the host institution and elsewhere, and the fellow has accepted a prestigious CERN fellowship.