7.1. Exotic physics

It is possible to add your own models to Sherpa in a straightforward way. To illustrate, a simple example has been included in the directory Examples/BSM/SM_ZPrime, showing how to add a Z-prime boson to the Standard Model.

The important features of this example include:

  • The Model.C file.

    This file contains the initialisation of the Z-prime boson and the definition of the Z-prime boson’s interactions. The remaining physics settings are inherited from the internal standard model implementation. The properties of the Z-prime are set here, such as mass, width, electromagnetic charge, spin etc as well as the right- and left-handed couplings to each of the fermions are set there.

  • An example Makefile.

    This shows how to compile the sources above into a shared library.

  • The example run card Sherpa.yaml. Note in particular:

  • The line SHERPA_LDADD: SherpaSMZprime in the config file.

    This line tells Sherpa to load the extra libraries created from the *.C files above.

  • The line MODEL: SMZprime in the config file.

    This line tells Sherpa which model to use for the run.

  • The following lines in the config file:

    PARTICLE_DATA:
  32:
    Mass: 1000
    Width: 50

    These lines show how you can overrule the choices you made for the properties of the new particle in the Model.C file. For more information on changing parameters in Sherpa, see Input structure and Parameters.

  • The lines

    Zprime:
  Zp_cpl_L: 0.3
  Zp_cpl_R: 0.6

    set the couplings to left and right handed fermions.

To use this model, create the libraries for Sherpa to use by running

$ make

in this directory. Then run Sherpa as normal:

$ ../../../bin/Sherpa

To implement your own model, copy these example files anywhere and modify them according to your needs.

Note: You don’t have to modify or recompile any part of Sherpa to use your model. As long as the SHERPA_LDADD parameter is specified as above, Sherpa will pick up your model automatically.

Furthermore note: New physics models with an existing implementation in FeynRules, cf. [CD09] and [CdAD+11], can directly be invoked using Sherpa’s support for the UFO model format, see UFO Model Interface.