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/Models/SM_ZPrime, showing how to add a Z-prime boson to the Standard Model.

The important features of this example include:

  • The SM_Zprime.C file.

    This file contains the initialisation of the Z-prime boson. The properties of the Z-prime are set here, such as mass, width, electromagnetic charge, spin etc.

  • The Interaction_Model_SM_Zprime.C file.

    This file contains the definition of the Z-prime boson’s interactions. The right- and left-handed couplings to each of the fermions are set here.

  • An example Makefile.

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

  • The line SHERPA_LDADD: SMZprime 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 SM_Zprime.C file. For more information on changing parameters in Sherpa, see Input structure and Parameters.

  • The lines Zp_cpl_L: 0.3 and Zp_cpl_R: 0.6 set the couplings to left and right handed fermions in the config file.

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.