# 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,α_{s,}
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

*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 α_{s}
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
λ_{QCD},
which can then be turned to an improved prediction α_{s}
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, α_{s}.

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