OpenLoops+Sherpa

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.

Installation

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:

autoreconf -i
./configure --enable-openloops=PATH_TO_OPENLOOPS [other Sherpa configure options]
make
make install

The OpenLoops installation path can be modified at runtime by setting (in the Sherpa run card or command line):

OL_PREFIX=PATH_TO_OPENLOOPS

Note, that all required process libraries have to be downloaded and compiled before any Sherpa+OpenLoops run.

Usage

An example run card to use OpenLoops+Sherpa can be found within the installation of Sherpa in the file

PATH_TO_SHERPA/AddOns/OpenLoops/example/Run.dat

This example can be run via

cd PATH_TO_SHERPA/AddOns/OpenLoops/example/
PATH_TO_SHERPA/bin/Sherpa -f Run.dat

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

Loop_Generator OpenLoops;

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;

NLO_QCD_Mode MC@NLO;

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 ... ;