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SARAH
SARAH

Particles.m

Last edited by Martin Gabelmann
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Particles.m

General

The particle file contains information about all fields of the model. This information is usually needed needed for an appropriate output.

  • Description: a string for defining the particle.
  • PDG: defines the PDG numbers of all generations.
  • PDG.IX: defines a nine-digit number of a particle supporting the proposal Ref. . By default, the entries of PDG are used in the output (to switch to the new scheme, either at the beginning of a SARAH session or in the model files, one has to set UsePDGIX = True;).
  • ElectricCharge: defines the electric charge of a particle in units of e. This information is exported to the CalcHep /CompHep and UFO model files.
  • Width: can be used to define a fixed numerical value for the width.
  • Mass: defines, how MC tools obtain a numerical value for the mass of the particle:
    • a numerical value can be given
    • the keyword Automatic assigns that SARAH derives the tree level expression for the mass from the Lagrangian. The mass is then calculated by using the values of the other parameters.
    • the keyword LesHouches assigns that this mass is calculated numerically by a spectrum generator like SPheno and provided via a Les Houches spectrum file. This is usually the preferred method because also loop corrections are included.
  • OutputName: defines the name used in the different outputs for other codes.
  • LaTeX: defines the name of the particle used in the LaTeX output.
  • FeynArtsNr: the number assigned to the particle used in the FeynArts output
  • LHPC: defines the column and colour used for the particle in the steering file for the LHPC Spectrum Plotter. All colours available in gnuplot can be used.
  • Goldstone: for each massive vector boson the name of corresponding Goldstone boson is given.

The properties of all particles appearing either as gauge or mass eigenstates, or just at intermediate steps can be defined in particles.m. Usually, the user is only interested in the output for the mass eigenstates. Therefore, it is not really necessary to define the entire properties of all intermediate states and the gauge eigenstates. The only input which is usually helpful to have a nice looking LaTeX output is to define the LaTeX syntax for all particles which appear at any stage in the model.

Considered eigenstates

This information must be given for all eigenstates in arrays named

ParticleDefinition[$EIGENSTATES]

Only for the LaTeX output also the names of the Weyl spinors and intermediate states (like scalar and pseudo scalar components of Higgs) should be given to improve the look and readability of the produced pdf file. For this purpose, the list

WeylFermionAndIndermediate

can be used.

Example

For the eigenstates after EWSB, an entry might look like

ParticleDefinitions[EWSB} = {
   ...
   {Sd ,  {  Description -> "Down Squark",
             PDG ->  {1000001,2000001,1000003,2000003,1000005,2000005},
             PDG.IX -> {-200890201,-200890202,-200890203,-200890204,-200890205,-200890206}
             Mass -> Automatic,
             FeynArtsNr -> 14,
             LaTeX -> "\\tilde{d}",,
             ElectricCharge -> -1/3,
             LHPC -> {7, "cyan"},
             OutputName -> "sd" }},
   ... }

Global definitions

It is also possible to define global properties for parameters or particles which are present in more than one model file. These properties are afterwards used for all models. The global definitions are saved in the files particles.m and parametes.m directly in the main model directory. For each parameter or particle, an entry like

{{        Descriptions -> "Down Squark",
          PDG ->  {1000002,2000002,1000004,2000004,1000006,2000006},
          Width -> Automatic,
          Mass -> Automatic,
          FeynArtsNr -> 13,
          LaTeX -> "\\tilde{u}",
          OutputName -> "um" }},

can be added. In particular, the entry Description is important. This should be an unique identifier for each particle or parameter. This identifier can later on be used in the different files of the different models, e.g.

{Su ,  {  Descriptions -> "Down Squark"}},

Of course, it is also possible to overwrite some of the global definitions by defining them locally, too. For instance, to use u instead of um as output name in a specific model, the entry should be changed to

{Su ,  {  Descriptions -> "Down Squark",
          OutputName -> "u" }},

in the corresponding particle file of the model.

See also