The increase of complexity in the scattering processes that are of
interest at the next generation of colliders calls for new tools for
automatic computation and event generation. Many different problems
have to be tackled simultaneously: The program has to be versatile and
necessarily involves a large degree of automatization since the list
of multi-particle processes to be considered is much longer than can
be included in a hard-coded process library like
PYTHIA [1]. The precision required for the
predictions makes the traditional distinction between signal and
background processes obsolete, since interference effects often cannot
be neglected. Thus, massive multi-particle phase space has to be
handled in the presence of many resonances and nearby singularities.
Since detector effects need to be studied, the program has to have a
convenient user interface, and it must be able to generate unweighted
events with reasonable efficiency.
During the workshops of the ECFA/DESY study for a future Linear
Collider it became obvious that no single existing package was able to
meet all these needs. Therefore, the initial idea of WHIZARD was to
combine known packages for generating matrix elements with a program
which is able to treat generic phase space, integrate and generate
events. The matrix element packages included are
CompHEP [2], MadGraph [3], and
O'Mega [4], which together cover the whole set of processes
that currently can be handled automatically (at tree level). The
latter problem could be solved with the help of the new
VAMP [5] integration program, which extends the VEGAS
algorithm to multi-channel parameterizations and thus makes it
possible to handle the complex singularity patterns of multi-particle
phase space in a uniform way.
The task for WHIZARD was to provide the actual phase space
parameterizations, Jacobians and transformations, provide a consistent
environment and to make the programs communicate with each other by
common interfaces. The user had to be given a simple setup with
common configuration and parameter definition files, commands to run
all programs consistently without the need for manual intervention (a
simple make should suffice), and an analysis system which
allows for rapid inspection of the results as well as for interfacing
hadronization and detector simulation programs. The program had to
keep full track of beam polarization and include beamstrahlung and
initial-state radiation. For hadronization, an interface to external
programs should be included. Finally, it should allow for flavor
summation in the final state: usually, many different processes
contribute to a single final state that cannot be distinguished
experimentally, and thus should be covered in a single run.
These goals have to a large extent been achieved by the current
release. However, there is still room for improvement. Most
prominent, the treatment of QCD effects is incomplete. This is rather
a problem of the matrix element generators and the hadronization
models which do not treat interfering color structures correctly in
all cases. Furthermore, these programs obviously limit WHIZARD to
the physics models they can support.
The acronym WHIZARD stands for
W, HIggs, Z, And
Respective Decays
which is the class of processes (electroweak processes at e+e-
colliders) the program was originally designed for. However, the
present version has a much larger range of applicability.