Project

Discipline

elementary particle physics; weak interaction; electroweak standard model; electroweak radiative corrections; NLO calculation; gauge-boson production; jet production; generalized unitarity

Lead
Lay summary
In the near future the Large Hadron Collider (LHC) will start operation. It will allow to study the fundamental interactions of matter at a new energy scale. Presently, the known interactions between elementary particles are well described by the Standard Model. In order to exploit the LHC data, precise theoretical predictions for the observable reactions are mandatory. Comparing these predictions with experimental data allows to verify so far untested parts of the Standard Model or to uncover new phenomena. This project aims at improving the methods needed to perform the required theoretical calculations.Since the Standard Model cannot be solved exactly, predictions for particle reactions at high-energy colliders are obtained within perturbation theory. Predictions in leading perturbative order suffer from large uncertainties, and a decent knowledge can only be obtained by including at least the next-to-leading-order perturbative corrections. While the corrections due to the strong interaction yield the largest effects, also the corrections associated with the electroweak interaction are required. Many interesting processes at the LHC involve many external particles for which corrections are difficult to calculate. Traditional methods based on Feynman diagrams have been constantly improved, and the technical frontier has been pushed to processes with four particles in the final state. An alternative promisin method using so-called generalized unitarity has been developed in recent years. So far, almost all developments and applications using this new method focus on corrections due to the strong interaction. The core of this research proposal is the application of the generalized-unitarity method to the calculation of electroweak corrections and the development of suitable numerically stable and efficient algorithms. The much richer structure of electroweak corrections leads to additional complications. Moreover, we plan to improve the generalized-unitarity framework in different aspects. We intend to apply the new method to the calculation of the electroweak corrections for gauge-boson production in association with 2 jets. This process has a large cross section, serves as calibration process, and is a background to many new-physics signals.This project will contribute to the improvement of calculational techniques required to exploit the wealth of data to be obtained by the LHC. If the generalized-unitarity method turns out to be superior to the Feynman-diagrammatic method this project will establish a new method for such calculations which will be widely used for future calculations of scattering processes at high-energy colliders.

Direct link to Lay Summary

Last update: 21.02.2013

Number	Title	Start	Funding scheme

In the near future the Large Hadron Collider (LHC) will start to collect data at the TeV scale. To explore this energy regime one needs adequate theoretical predictions both for signal and background processes. At a hadron collider, where the strong interaction is omnipresent, leading-order cross sections suffer from large uncertainties, and a decent knowledge can only be obtained by including at least the next-to-leading-order (NLO) QCD corrections. For many reactions at the LHC even higher-order QCD and NLO electroweak corrections are required for a sufficiently accurate prediction. Electroweak corrections are particularly important for processes with large cross sections and high relevant centre-of-mass energies.Many interesting processes at the LHC involve multiparticle final states for which NLO corrections are difficult to calculate. Traditional methods based on Feynman diagrams have been constantly improved in recent years, and the technical frontier has been pushed to processes with four particles in the final state. An alternative method using generalized unitarity has been developed in recent years and looks very promising for NLO corrections to multiparticle processes. So far, almost all developments and applications within the generalized-unitarity approach focus on NLO QCD.The core of this research proposal is the application of the generalized-unitarity method to the calculation of NLO electroweak corrections and the development of suitable numerically stable and efficient algorithms. Compared to QCD corrections, electroweak corrections confront us with additional complications: they typically involve a larger number of contributions, depend on many more parameters, involve complicated thresholds and pose more severe problems for numerical stability.In this context, we plan to improve the generalized-unitarity framework in different aspects: first, the improvement of the numerical stability for phase-space regions where Gram determinants of external momenta become small or vanishing, which occurs frequently in electroweak NLO corrections to multiparticle processes, is mandatory. In the existing implementations of the generalized-unitarity framework these problems are circumvented by moving from double to quadruple precision or even higher, paying a large price in computation time. We plan instead to transfer our solutions from the conventional formalism. Furthermore, we will study whether our experience from the Feynman-diagrammatic formalism can be used to simplify the determination of rational terms in the generalized-unitarity framework.We intend to apply the new method to the calculation of the electroweak corrections for a non-trivial, phenomenologically relevant LHC process, namely gauge-boson production in association with 2 jets. This process has a large cross section, serves as calibration process for Higgs production in vector-boson fusion, and is a background to many new-physics signals. For large transverse momenta we expect electroweak corrections of several tens of per cent. If the generalized-unitarity method turns out to be superior to the Feynman-diagrammatic method for multiparticle NLO electroweak corrections this project will establish a new method for such calculations in the Standard Model and its extensions. Like for other methods developed by the applicant in the past, we expect that the results of this project will be widely used in the particle physics community.