.. _Integration:
***********
Integration
***********
The following parameters are used to steer the integration:
.. contents::
:local:
.. _int_INTEGRATION_ERROR:
INTEGRATION_ERROR
=================
.. index:: INTEGRATION_ERROR
Specifies the relative integration error target.
.. _int_INTEGRATOR:
INTEGRATOR
==========
.. index:: INTEGRATOR
Specifies the integrator. The possible integrator types depend on the
matrix element generator. In general users should rely on the default
value and otherwise seek the help of the authors, see :ref:`Authors`.
Within AMEGIC++ the options ``AMEGIC: {INTEGRATOR: ,
RS_INTEGRATOR: }`` can be used to steer the behaviour of the
default integrator.
* :option:`4`: building up the channels is achieved through respecting
the peak structure given by the propagators. The algorithm works
recursively starting from the initial state.
* :option:`5`: this is an extension of option 4. In the case of
competing peaks (e.g. a Higgs boson decaying into W+W-, which
further decay), additional channels are produced to account for all
kinematical configurations where one of the propagating particles is
produced on its mass shell.
* :option:`6`: in contrast to option 4 the algorithm now starts from
the final state. The extra channels described in option 5 are
produced as well. This is the default integrator if both beams are
hadronic.
* :option:`7`: Same as option :option:`4` but with tweaked
exponents. Optimised for the integration of real-subtracted
matrix-elements. This is the default integrator when at least one of
the beams is not hadronic.
In addition, a few ME-generator independent integrators have been
implemented for specific processes:
* :option:`Rambo`: RAMBO :cite:`Kleiss1985gy`. Generates isotropic
final states.
* :option:`VHAAG`: Vegas-improved HAAG integrator
:cite:`vanHameren2002tc`.
* :option:`VHAAG_res`: is an integrator for a final state of a weak
boson, decaying into two particles plus two or more jets based on
HAAG :cite:`vanHameren2002tc`. This integrator can be further
configured using ``VHAAG`` sub-settings, i.e. ``VHAAG:
{: ``}. The following sub-settings are
available. ``RES_KF`` specifies the kf-code of the weak boson, the
default is W (``24``). ``RES_D1`` and ``RES_D2`` define the
positions of the Boson decay products within the internal naming
scheme, where ``2`` is the position of the first outgoing
particle. The defaults are ``2`` and ``3``, respectively, which is
the correct choice for all processes where the decay products are
the only not strongly interacting final state particles.
.. _VEGAS_MODE:
VEGAS_MODE
==========
.. index:: VEGAS_MODE
Specifies the mode of the Vegas adaptive integration. :option:`0` disables
Vegas, :option:`2` enables it (default).
.. _FINISH_OPTIMIZATION:
FINISH_OPTIMIZATION
===================
.. index:: FINISH_OPTIMIZATION
Specifies whether the full Vegas optimization is to be carried out.
The two possible options are :option:`true` (default) and :option:`false`.
.. _PSI:
PSI
===
.. index:: PSI
The sub-settings for the phase space integrator can be customised as follows:
.. code-block:: yaml
PSI:
:
# more PSI settings ...
The following sub-settings exist:
``NMAX``
The maximum number of points before cuts to be generated during integration.
This parameter acts on a process-by-process basis.
``ITMIN``
The minimum number of points used for every optimisation cycle. Please note
that it might be increased automatically for complicated processes.
``ITMAX``
The maximum number of points used for every optimisation cycle. Please note
that for complicated processes the number given might be insufficient for
a meaningful optimisation.
``ITMIN_BY_NODE``
Same as ``ITMIN``, but specified per node to allow tuning of
integration performance in large-scale MPI runs.
``ITMAX_BY_NODE``
Same as ``ITMAX``, but specified per node to allow tuning of
integration performance in large-scale MPI runs.