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From Structures to Predictions: Precision Multi-jet QCD for LHC Physics

Final Report Summary - PMJ-QCD (From Structures to Predictions: Precision Multi-jet QCD for LHC Physics)

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Overview
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The experiments at the Large Hadron Collider (LHC) at CERN, Geneva, give rise to an unprecedented opportunity to understand Particle Physics in the high-energy regime. The discovery of new particles or even an extension of the current understanding of the fundamental forces is a likely outcome of the experiments. The early physics exploration has already culminated in the discovery of the Higgs particle. The discovery has lead to the Nobel Prize for P. Higgs and F. Englert in 2013. Not surprisingly, the award was given with a clear reference to the contributions of the ATLAS and CMS experiments at the LHC.

The physics exploration relies on the synthesis of theory predictions and the measurements of the ATLAS and CMS experiments. In particular, precise predictions based on the established Standard Model of particle physics (SM) play an important role. In the absence of such predictions, new physics signals may remain hidden, or backgrounds may be falsely identified as exciting new physics signals.

The research of my group contributes to the ongoing physics exploration at the LHC. We currently focus on the challenging computation of quantum corrections of SM predictions. The distinguishing feature of the research is the combination of formal research with timely phenomenological work. Research on the formal properties of quantum-field theory often allows to discover new ways to compute and to remove limiting bottlenecks. In addition, the formal research is exciting and fun leading to a deeper understanding of quantum-field theory.

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Research Contributions During Reporting Period
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(I) Formal methods in quantum field theory.
In order to match the expected precision of cross-section measurements new next-to-next-to-leading order (NNLO) QCD computations will be important. In the recent years, a number of impressive NNLO results for two-to-two processes have become available relying on conventional (often combinatorial) methods in quantum-field theory. The complexity of such computations is challenging and often forbidding, however, in many cases the final results turn out much simpler than the problematic intermediate expressions. We explore alternative computational methods which exploit universal physical properties (factorisation and unitarity) in order to streamline cross-section computations:

(a) I developed a new method for multi-loop computations in generic quantum-field theories, referred to as `numerical unitarity method' below. This method extends the established unitarity method and is suitable for an automated (exact) numerical implementation, but also allows for an analytic setup. In a purely numerical implementation the approach promises to reduce challenges of multi-scale computations. Recently my group published [2-3] the first computation of two-loop scattering amplitudes using the numerical unitarity approach. The related method at one-loop level set the state-of-the-art for high-multiplicity computations at one-loop.

(II) Precise predictions for SM processes with multi-particle final states.

Such final-state signatures are interesting per se and important backgrounds in
new physics searches. Multi-particle processes include high powers of the coupling constants, such that reliable predictions require the inclusion of quantum effects. In turn, these effects are challenging to obtain given the large number of physical scales involved.

(a) With our matrix-element generator BlackHat we hold the world record in particle multiplicity for the computation of next-to-leading order (NLO) quantum corrections in QCD. The most challenging process for the LHC setup includes six-particles in the final state []

(b) We obtained new predictions for the three important process classes including vector bosons and jets.

(III) Phenomenological studies for dedicated analyses of the LHC experiments.

Precise theory predictions are an important benchmark for standard-candle processes and, in addition, extend the reach of new physics searches. My research provided the following results including dedicated observables, in which corss section ratios are compared and analysed in detail. Furthermore, we constructed analysis formats, which allow to hand over our computations to the experiments at CERN.

(IV) Refined final-state description for signal processes.

Investigating the interplay of spin-correlations and interference effects often allows to define interesting observables which lift signals over backgrounds. My central contribution in this research direction has been the following: With my collaborators I predicted bounds on width of the SM Higgs boson based on studied the interference effects of the resonant The main results were obtained in a collaboration with the research groups at SLAC and the University of Buenos Aires.

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Future
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The long-term perspective of my research is set by the lifespan of the LHC. At the same time the research is very timely, given the expected data from the ATLAS and CMS experiments at the LHC during the coming decades. In addition, a number of formal research directions seem particularly interesting on the long run; these include research on theories of gravitation as well as the strong-coupling regime of (supersymmetric) quantum-field theories which I have already worked on during my PhD and my early postdoc time.

The Marie-Curie Career Integration Grant (CIG) has provided the necessary stability to setup a research group (teaching, management, funding applications etc.) as well as the financial means to boost international collaborations. Most importantly, the CIG funds allowed to hire students and facilitated international travel to collaborate and disseminate the research within Europe and in the international research community.