Project Area B: Predictions for High Energy Reactions

Among the major goals of the experimental programmes at high energy colliders, the elucidation of the mechanism of spontaneous symmetry breaking in the electroweak interactions and the search for physics beyond the Standard Model (SM) figure most prominently. In both cases, discovery requires a precise comparison of experimental results with theoretical predictions, within the SM and beyond: any signal of new particles like the Higgs boson or supersymmetric particles will be seen as an excess of events over SM backgrounds, which need to be known. A subsequent measurement of new particle properties is even more demanding on theoretical precision. At least as challenging for the precision of theoretical predictions is an improved measurement of the properties of the known heavy particles, like the W and Z or the top quark. Precision calculations require the evaluation of higher order corrections in perturbation theory for scattering processes with multiple external particles. But also the perturbative and non-perturbative input parameters, like the strong coupling constant,αs, quark masses, or the parton distribution functions underlying any hadron collider cross sections, need to be known with improved precision. Finally, fundamental questions in the description of unstable particles need to be resolved for these calculations, since almost all heavy particles are short-lived. Project Group B deals with these questions in five tasks:

B1 Precision predictions for massive particle production
B2 Lattice computation of input parameters of perturbative QCD
B3 Parton distribution functions on the lattice and in the continuum
B4 Production of unstable particles
B5 Precision calculations for Higgs and BSM physics at the LHC

For the high energy colliders LHC, Tevatron and a future ILC, precision predictions at the level of next-to-next-to-leading order (NNLO) are needed for the production cross sections of pairs of massive particles. Examples are final states like top-quark pairs, W+ W-, ZZ or W± Z at the LHC, or Bhabha scattering at an ILC. Project B1 aims at the calculation of total cross sections and of single differential distributions for these processes with NNLO accuracy, which should reduce theoretical uncertainties from unknown higher orders of the perturbative expansion to well below 10%. The systematic treatment of these corrections ranges from the computation of analytical expressions to numerical predictions for collider observables. The development of software tools for various parts of the calculation is an integral part of the project.

The cross sections for these pair production cross sections as well as for many other high energy processes are described by perturbative QCD in terms of the coupling αs and the quark masses as free parameters. Project B2 is studying the connection of these parameters to hadronic quantities, like meson and baryon masses, by lattice simulations. Hadron masses are measured very precisely and, thus, they can provide very accurate information on QCD input parameters in principle. Because of the non-perturbative character of the bound-state problems, such a program is best addressed by matching lattice calculations with perturbation theory. Intrinsically, this is a multi-scale problem which requires the development of special techniques. Additional simplifications, like unphysical quark masses, have been necessary in the past and are still required, but will be eliminated step by step. From comparison with measured mass values, the calculations will produce improved determinations of quark masses and λQCD, which can then be turned to an improved prediction αs at high energies.

Project B3 aims at improving our knowledge of parton distribution functions (PDFs), by combining perturbation theory, phenomenology and lattice simulations. The calculation of moments of PDFs on the lattice provides ab-initio information, once small enough quark masses can be simulated. On the other hand, the direct extraction of PDFs from precision data will be improved by pushing the needed perturbative calculations to 3-loop order. In order to achieve this, new calculational tools will be developed which can also be used in other multi-loop problems. The results will then be used to perform world data analyses of both unpolarised and polarised PDFs and to apply them to improved inclusive cross section predictions for the LHC. The direct comparison of moments of PDFs as determined from experiment with those calculated on the lattice, will provide interesting information on the validity range of the perturbative treatment. These comparisons will also provide improved information on other QCD parameters, like the strong coupling constant, αs.

The heavy particles mentioned above all decay rapidly, at time scales which are short compared to the QCD scale but long compared to their mass. The systematic treatment of such unstable particles (narrow resonances) in quantum field theories requires methods that go beyond the standard diagrammatic expansion in the coupling constant. The origin and solution of the problem lie in the existence of two separate scales, mass and width, which allow to construct an effective field theory. In project B4 the appropriate effective field theory has been developed and will be further refined. This effective field theory approach as well as the complex mass scheme are then employed in parallel to predict quantum corrections to the production and subsequent decay of unstable particles, primarily weak gauge bosons and top quarks, in high-energy collisions.

The discovery and interpretation of new physics at the LHC, for example Higgs production or supersymmetry, requires precise calculations for signal and background processes. In project B5 the radiative corrections to various production and decay processes of particular phenomenological relevance will be determined. On the one hand these are NLO QCD corrections to multi-parton processes like WWjj production. On the other hand, improved relations between particle masses and model parameters of SM extensions, like supersymmetric models, will be determined and thus provide the basis for a precise interpretation of experimental data. The NLO multi-parton cross sections will be further improved by matching them with existing parton shower programs, and also a better treatment of final state photons is planned, namely the collinear emission of photons from quarks will be modelled non-perturbatively with the help of fragmentation functions. The precise predictions will then form the basis of detailed phenomenological investigations in Higgs physics and of physics beyond the Standard Model.

Last change: 8th June 2011