OpenLoops can be used as a plug-in of Sherpa 2.1.0 or later. For the installation of Sherpa and the usage of Sherpa+OpenLoops please also refer to the Sherpa documentation.
OpenLoops must be enabled by configuring Sherpa with the '--enable-openloops' option. To this end the following commands must be executen in the Sherpa installation directory:
The OpenLoops installation path can be modified at runtime by setting (in the Sherpa run card or command line):
Note, that all required process libraries have to be downloaded and compiled before any Sherpa+OpenLoops run.
An example run card to use OpenLoops+Sherpa can be found within the installation of Sherpa in the file
This example can be run via
Additional example OpenLoops+Sherpa run cards can be found here.
In particular, to run Sherpa in combination with OpenLoops it is sufficient to add OpenLoops to the list of available Matrix element generators via the flag
ME_SIGNAL_GENERATOR Comix Amegic OpenLoops;
and at the same time to set in the processes section of the Sherpa run card the flag
Sherpa will now automatically use the one-loop matrix elements from OpenLoops when for example a NLO QCD fixed-order simulation is requested via (in the processes section of the run card)
NLO_QCD_Mode Fixed_Order;or a matched sample via
For details on these modes and many other options we refer to the Sherpa documentation.
In general Sherpa automatically handels all the necessary parameter initialization of OpenLoops. However, user-defined parameters can be passed from the Sherpa run card (or command line) to OpenLoops via
OL_PARAMETERS FIRST_PARAM_NAME FIRST_PARAM_VALUE SECOND_PARAM_NAME SECOND_PARAM_VALUE ... ;A list of possible parameters can be found here.