The Sherpa interface (by C. Duhr and S. Schumann)

Sherpa is a multi-purpose event generator that can be used to generate events in various physics models both at the parton and hadron level. The program can account for parton showering and hadronisation, and, where appropriate, also underlying events.

The Sherpa Monte Carlo is publicly available and can be obtained from ​www.sherpa-mc.de/. The interface to FeynRules will be available from version Sherpa-1.1.0 on. If you experience any problems with the FeynRules--Sherpa interface send an email to claude.duhr@… and sschuman@…

Generating a new Sherpa model with FeynRules

The FeynRules interface to Sherpa enables the user to implement his new physics model into Sherpa just by knowing the Lagrangian and the FeynRules model file. The interface is called via

WriteSHOutput[L1, L2, L3,...]

where L1, L2, L3, ... denote the Lagrangians which should be used to calculate Feynman rules from. Running the interface creates in the current working directory a new subdirectory !ModelNameSH, where ModelName is the name of the the FeynRules model file. This directory contains all the files needed for the implementation of the new model into Sherpa. These files follow a convention agreed on between the FeynRules and Sherpa authors to provide maximal possible generality and flexibility by being compliant with the Sherpa standards.

In particular the list of files includes:

• The central file called feynrules.dat that contains a list of variables representing the input files for the model parameters, particles and interactions, per default this files is called feynrules.dat. The list of input parameters and default settings reads, there meaning will be explained below
  • PARAMIDENT = ident_card.dat
  • PARAMCARD = param_card.dat
  • PARAMDEFINITION = param_definition.dat
  • PARTICLES = particles.dat
  • INTERACTIONS = interactions.dat
• In the file identified through PARAMIDENT all the external parameters of the model are listed by name and it is stated on which position they can find in correponding blocks in the PARAMCARD file.
• The PARAMCARD file is a SUSY Les Houches style file that contains block statements from that the numerical values of all the external parameters are inferred
• In the file associated with PARAMDEFINITION the definitions of all relevant internal parameters, as functions of the external parameters and previously defined internal parameters of the model, are listed.
• The file tagged by PARTICLES has to contain a list of all the particles occurring in the considered model. Particles are identified through there PDG Monte Carlo code (where existent) and their mass, width, U(1) charge, weak isospin, SU(3) charge, spin, and possibly self-conjugate character has to be specified.
• INTERACTIONS specifies the file where all the interaction vertices of the model get listed. For example the three-gluon vertex is represented through {{{ VERTEX 21 21 21 # gluon -> gluon gluon
  1. G # right-handed coupling
  2. G # left-handed coupling
  3. F[1,2,3] # SU(3) colour structure
  4. Gauge3 # Lorentz structure }}}

To run Sherpa with the newly defined FeynRules model the masterfile feynrules.dat and all the other model input files have to reside in the directory specified by the PATH variable for your Sherpa call, e.g. './Sherpa PATH=MyFeynrulesSetup/' where besides the FeynRules output files all the other Sherpa input files that fully specify the generator run have to reside. From these files two have to be edited. In Model.dat the switch MODEL has to be set to

MODEL = FeynRules

and the variable GENERATOR_ON has to be set equal to 1. In addition the variable FEYNRULESINPUT has to be set to

FEYNRULESINPUT = feynrules.dat

the master file. The last step is to edit ME.dat and set SIGNAL_MODEL equal to

SIGNAL_MODEL = FeynRules

Features and limitations

• The core of Sherpa builds its matrix element generator Amegic++ that automatically constructs tree-level amplitudes for arbitrary processes in a given physics model, specified through its parameters, particles and interactions. Guided by the contributing Feynman diagrams automatically suitable phase integrators are constructed.
• Sherpa/Amegic++ can treat 3 and 4 point interactions of spin-0, spin-1/2, spin-1 and spin-2 particles. However, there maybe some Lorentz structures missing that are not present in the MSSM or the ADD model of large extra dimensions. Whenever such a vertex appears in the new FeynRules model the program will throw an exception.
• Similarly the colour algebra of Sherpa is at present limited to %u03B4(i,j), %u03B4(A,B), T(A,i,j), f(A,B,C) and d(A,B,C), where i and j label colour triplets and A,B,C colour octets. Whenever there appear other colour coefficients the program will throw an exception.
• It should be noted that there may be limitations concerning the simulation of parton showers of new coloured states or the fragmentation routines when new SU(3) coloured states are considered.

Citation

When you use Sherpa please cite T. Gleisberg et al., JHEP 0402 (2004) 056 ​hep-ph/0311263.