Research Program - Overview
Computational physics has been established as an independent field of research some time ago. Traditionally, theoretical particle physics has notably profited from the development of computer-based techniques. In turn, the challenges provided by complex particle physics phenomena have led to new developments in computational physics.
The two central themes of this CRC/TR, the perturbative calculation of high-energy scattering amplitudes using computer algebra and the numerical simulation of quantum field theories on the lattice, are certainly outstanding examples for this fruitful interplay. Both approaches were initiated by Nobel prize laureates (M. Veltman, K. Wilson) and were explicitly mentioned by the Nobel committee as milestones in particle physics. Recently, the rapid increase in computer resources and substantial algorithmic improvements have considerably extended the research prospects. For example, including dynamical fermions in lattice simulations allows to predict observables, e.g. transition matrix elements or particle spectra, with an unprecedented precision such that the results can be expected to be fully realistic for the first time. On the other hand, computer algebra has offered the opportunity to perform fully automated perturbative calculations at the three- or four-loop level and, hence, push the precision of the corresponding predictions to a new level. Moreover, automated one-loop corrections to processes with many particles in the final state have recently attracted a lot of attention given the demands of physics at the Large Hadron Collider (LHC).
The CRC/TR 9 aims at two important goals. On the one hand, we work on the efficient combination of the methods of perturbative quantum field theory and lattice gauge theory. On the other hand, we develop calculational and mathematical tools and provide necessary predictions for the collider experiments of the coming years, in particular the LHC. These results together contribute to the determination of fundamental parameters and/or the identification of completely new phenomena with increased accuracy.
Progress in the different research areas has always been and still is motivated by the physical problems, experimental results, and the theoretical investigations as proposed here. A deep understanding of the Standard Model (SM) of particle physics --- including QCD, electroweak interactions, and flavour dynamics --- is essential for the interpretation of existing data as well as for the predictions for new experiments. In particular, with the start of the Large Hadron Collider (LHC) in 2009, the high energy frontier is finally pushed up by a factor of five to ten and reaches the TeV scale. The third funding period of this CRC/TR coincides with the period when the first substantial physics results from the LHC are expected. It is possible that the landscape of particle physics at the high-energy frontier will look quite different at the end of this period from what we imagine today. By devising strategies to improve the accuracy of SM parameters, calculating scattering processes precisely and exploring the signatures of extensions of the SM, the research programme of this CRC/TR contributes to the foundations upon which the interpretation of new phenomena in the experimental data will rest. In addition, new physics can be explored by precision measurements and precision calculations of electroweak phenomena and flavour physics as also pursued in this CRC/TR. In principle, the precise determination of parameters could lead to an understanding of the fundamental interactions up to the "GUT" scale, i.e. up to energies exceeding even the LHC energy by more than ten orders of magnitude.
The members of this CRC/TR work on a broad selection of topics in theoretical particle physics, including perturbative quantum field theory, flavour physics, heavy quark physics, lattice gauge theory, and computer algebra, with a focus on precision calculations and computational physics. The research on hadron-collider physics and on physics beyond the standard model has been intensified in the first as well as in the second funding period --- by appointing new project leaders, including new projects, and by shifting the focus of existing research projects. Within the CRC/TR 9, all these topics are investigated employing the combined expertise of its members. Problems in flavour physics, e.g. B-meson decays, are tackled with methods of perturbative quantum field theory as well as with lattice simulations in a common project. The results may contribute to answer the fundamental question about the origin of flavour violation. A common project on parton distribution functions, which combines lattice gauge theory and perturbative QCD, addresses the computation of the hadron structure from first principles, i.e. the QCD Lagrangian itself. New findings on complicated decay and scattering amplitudes are expected from combining computer algebra, parallel computation, and innovative mathematical methods. Important applications arise in Higgs physics or generically in the investigation of the spontaneous breaking of electroweak symmetry. The determination of the strong coupling constant and the quark masses both from high-energy data, using perturbative methods, and low-energy data, using lattice gauge theory, is still of special importance. Moreover, also the development of new methods is pursued, e.g. different lattice discretisations, effective field theories, and automation tools for Feynman diagram computations, providing the basis for the subsequent calculation of a wide range of high-energy physics phenomena.
Long-term goals of the CRC
Current and new experiments will continue to measure the masses, lifetimes, and the interactions of elementary particles with increased accuracy. The LHC will allow to access the TeV scale directly and is designed to reveal the nature of electroweak symmetry breaking (EWSB). The LHC could also discover completely new particles and interactions, foremost the Higgs boson. These experimental results, in combination with theoretical input, will shed light on the structure of particles and interactions at the smallest distances ever accessed and form the basis for extrapolations to energy regimes which will never be accessible by direct collider experiments. The CRC/TR~9 plays an important role in providing this theoretical input.
The increasing energy leads to an increasing complexity of the final states at high-energy colliders. In addition, the statistics of present experiments such as LHC and B-factories allow for highly precise measurements, sensitive to the effects of quantum fluctuations, which must be matched by the theoretical accuracy. In combination, both aspects --- complex final states and highest demands on accuracy --- are the central features of research on quantum field theory in the next years and will be decisive for the scientific success of the experiments. These two aspects also define the central themes of this CRC/TR.
By combining the methods of perturbative quantum field theory, lattice calculations, techniques of mathematical physics, computer algebra, and numerical simulations the CRC/TR~9 focuses on the central topics of particle physics phenomenology as described above. In the following we list the goals of the CRC/TR~9 more specifically. Some of them were defined when this CRC/TR was first proposed, others have been added during the past eight years as the focus of our research evolved, as did the field as a whole.Fundamental parameters and quantum fluctuations
The precision of predictions for high-energy collisions will increase; the fundamental parameters, such as the strong coupling and quark masses, parton distribution functions, and CKM matrix elements in flavour physics will be determined with improved accuracy by combining perturbative calculations and lattice simulations. The complicated, short-lived quantum fluctuations, often visualised by Feynman diagrams and identified as loop effects, give access to energy scales far beyond the reach of present or future earth-bound colliders. An outstanding recent example of the relevance of quantum fluctuations was the indirect determination of the top-quark mass at 180~GeV before it was discovered at the Tevatron-Collider by direct production. Today, the limits on the Higgs-boson mass provide another example. Including quantum fluctuations in precise physical predictions requires extensive theoretical calculations. In the CRC/TR~9 we
- determine the strong coupling αs with improved accuracy of 1% from electron-positron annihilation and tau decays; deep-inelastic scattering world data; and hadronic quantities by perturbative methods and progressively realistic non-perturbative simulations (A1, B2, B3).
- determine the masses of the strange, charm, bottom and top quarks from multi-loop calculations of spectral functions in electron-positron annihilation; first principle calculations in lattice QCD; and pair-production cross sections (A1, B1, B2, C1, C3).
- reach precise theoretical predictions for QCD effects in flavour physics, in order to reduce the theoretical uncertainties in the determination of the CKM matrix by a combination of non-perturbative lattice and factorisation methods (C1).
- describe deep-inelastic scattering and determine the parton distribution functions from first principles by the combination of lattice simulations, and perturbative calculations at NNLO and N3LO and move to the calculation of anomalous dimensions and coefficient functions to order αs4 (B3, A1).
- achieve high precision predictions for the lightest Higgs boson mass and couplings in the MSSM and NMSSM (C5, B5).
The perturbative treatment of the electroweak and strong interactions has proven to be the ideal method for the description of scattering processes at the level of leptons, quarks, and gluons in very high-energy collisions. The large available energy leads to complex final states at the Tevatron (FNAL), the LHC (CERN) or a future linear collider (ILC). Final states with four or more particles and with quark or gluon jets have to be calculated. Loop corrections (NLO corrections) are essential in most cases to obtain trustworthy results. In the CRC/TR~9 we develop calculational tools and provide necessary predictions for these collider experiments, in particular the LHC. We contribute to the precision calculations of backgrounds as well as to the identification of new particles. The main goals are
- to explore the Higgs boson and new physics in the first period of LHC data taking (B5).
- to predict the SM backgrounds at the LHC with optimal precision (B5).
- to calculate precisely the cross section of pair production and single production of top quarks, and to investigate its couplings to other heavy particles (C4, B1, C3).
- to implement NLO QCD calculations as Monte Carlo programs which are interfaced with parton shower programs such as Herwig, PYTHIA or dedicated NLL parton showers (B5).
- to establish precise relations between the parameters at the electroweak and GUT scale and to explore the resulting restrictions on GUT models (C5).
- to provide the theoretical predictions to determine the luminosity at the ILC at the level of 10-4 by completing the calculation for the Bhabha scattering cross section at the two-loop order (B1).
New concepts in quantum field theory, advances in computer-algebra methods, and drastically improved numerical simulations of quantum chromodynamics on the lattice, along with the rapid progress in hardware development, will promote our understanding of the fundamental interactions. The CRC/TR~9 has a strong focus on developing the computer-algebra tools and quantum-field theory methods that allow to push calculations to a new quantitative level. We
- develop efficient and flexible computer-algebra systems to handle complicated Feynman diagram calculations; and provide and improve open source software for the analytic and numerical computation of the corresponding integrals (A2, B1).
- develop a fully automated NLO/NLL Monte Carlo event generator (A5).
- reach fully realistic lattice calculations by including the strange and charm degrees of freedom into lattice simulations and by driving the quark mass down to reach the experimentally measured pion mass (A4, B2, C1).
- develop new methods and tools for the efficient computation of massive 3-loop diagrams with one differential variable; new summation and integration technologies; and the systematic creation of special functions to compute Feynman diagrams analytically (B3).
- develop an effective field theory method to systematically account for the width of unstable particles in their production processes (B4).
- evaluate five-loop anomalous dimensions and the β function in QCD; test conjectures based on the AdS/CFT correspondence in N=4 supersymmetric gauge theory (A1).
Last Change: June 7th 2011