Today, the Standard Model (SM) of particle physics provides a thoroughly tested framework for describing electromagnetic, weak and strong interactions of the fundamental constituents of matter. The SM successfully describes all presently observed electroweak and strong interactions of matter particles (quarks and leptons) and of the mediators of the fundamental forces (photon, W and Z bosons, and the gluon). However, due to apparent shortcomings of the SM (e.g., hierarchy problem, fine tuning, absence of gauge-coupling unification at high energies), it is commonly believed that it is merely the low-energy limit of a more fundamental theory, which has additional symmetries such as Supersymmetry (SUSY), an additional symmetry that connects fermions and bosons or which has extra spatial dimensions. The main goal of present and future collider experiments is to further test the SM and to detect signals of new physics, either directly (through the production of non-SM particles), or indirectly (as small deviations from the predicted properties of SM particles). The PIs propose to investigate possible deviations from the SM in the interactions of top quarks and Higgs bosons, and to develop improved theoretical calculations and computational tools which are needed to perform precision studies of SM and SUSY particles at the Fermilab Tevatron, the CERN Large Hadron collider (LHC), and a future International e+e− Linear Collider (ILC). In order to extract precise physics information from these experiments, theoretical predictions must match or exceed the experimental accuracy. In light of the anticipated experimental accuracy, current predictions must be improved. This will involve first the calculation of radiative corrections and then their implementation in Monte Carlo event generators for realistic simulations. The broader impact of the proposed activities is that the research will contribute to an improved understanding of the properties of elementary particles and their interactions. The tools developed will be made publicly available and will help the experimental high-energy physics community to fully exploit the potential of present and future collider experiments for testing the SM and searching for signals of new physics. This project will provide an excellent training ground for students at both the undergraduate and graduate level, teaching them skills valuable beyond the scope of particle physics research. The proposed activities will be pursued in collaboration with scientists worldwide, partly in context of the American Linear Collider Physics Group, and will further US-international joint scientific efforts.
Between October 2008 and September 2012, this award funded research in theoretical particle physics conducted by Professor Ulrich J. Baur (until November 2010, deceased), Professor Doreen Wackeroth, one Postdoctoral Research Associate, and four graduate students. Particle physics research has entered an exciting era: Experiments at the CERN Large Hadron Collider (LHC) are exploring the fabric of matter at an unprecedented level of precision and are expected to provide answers to some of the most fundamental questions in science: What is responsible for the generation of mass and what is the nature of dark matter in the universe? The recent discovery of a new boson with Higgs-like properties at the LHC marks the beginning of an exciting journey where the nature of the mechanism responsible for the generation of mass and its messenger, the Higgs boson, is revealed. Besides the study of the new boson at the LHC and a possible future international electron-positron linear collider (ILC), these collider experiments strive to discover new particles and to gain new insights in the fundamental principles that govern all dynamics and properties of matter, i.e. beyond what is described by the Standard Model (SM) of particle physics. The SM is a thoroughly tested framework for describing electromagnetic, weak and strong interactions of the fundamental constituents of matter, based on a symmetry principle and mathematically formulated as a renormalizable Quantum Field Theory. The SM successfully describes all presently observed electroweak and strong interactions of matter particles (quarks and leptons) and of the mediators of the fundamental forces (photon, W and Z bosons, and the gluon). Despite this enormous success of the SM, it is generally accepted that the SM is merely a low-energy approximation to a more fundamental theory, which is expected to reveal itself at the LHC or at future high-energy experiments, such as the ILC, in form of the emergence of new, non-SM particles and interactions. A promising candidate for a theory beyond the SM, which also provides a dark matter candidate, is Supersymmetry (SUSY), an additional symmetry connecting fermions and bosons. The LHC is presently searching for signals of SUSY, and already succeeded in excluding a wide range of possible manifestations of SUSY. While direct signals of new particles may require collider energies not yet accessible, it is possible that new physics manifests itself first in form of minute deviations between measurements and equally precise predictions of properties of SM particles due to the indirect (virtual) presence of new particles in quantum-loop corrections. This project mainly contributed to the latter strategy in the quest of finding signals of physics beyond the SM. One necessary ingredient to the success of these high-energy collider experiments is the availability of calculations that predict the outcome of the collision experiment based on the underlying theory, which can then be confronted with and tested by collision data. This project investigated possible deviations from the SM in the interactions of top quarks and developed improved theoretical calculations and computational tools which are needed to perform precision studies of SM particles at the LHC (and the Fermilab Tevatron) and a future ILC. As a result of this project, new and improved calculations have become available for LHC studies involving top quarks, W and Z bosons and the Higgs boson, and ILC studies of Higgs interactions. These improvements and studies have been necessary in order to fully exploit the potential of the LHC (and an ILC) for unraveling the origin of electroweak symmetry breaking and mass generation, and to search for and disentangle signals of SUSY. Specifically, it involved complex quantum-field theoretical calculations of cross sections at higher-order in perturbation theory and their implementation in Monte Carlo computer programs. Most programs have been made publicly available and results have been presented at national and international conferences and in peer-reviewed journal publications (as well as produced upon request, in particular for the Higgs boson search at the LHC). Predictions provided by these Monte Carlo programs have been directly used in the interpretation of LHC data. These calculations have been challenging and have provided the graduate students involved with an advanced research experience and with skills valuable beyond the scope of particle physics research.
